Why is Christianity considered a monotheistic religion?

"place no other Gods before me"

I understand this to mean "do not worship any other Gods in place of me"

To answer your questions, it is necessary to define what God is and then describe what God is to us, His creation.

God is the originator, the creator, the sustainer. He comes from nothing and nothing comes from him (directly). He need only say 'be' and it is.

He alone is worthy of worship and to Him alone we must turn to for help.

This is the true nature of Montheism i.e. One cannot worship or pray for assistance to anything other than God.

By this definition of unitarian monotheism (the true form of the faith of Abraham), popular Christianity is not monotheistic and it cannot claim to be.

It is in fact polytheistic if it subscribes to the belief in the shield model (below).

It encourages worship of a holy trinity in addition to God and prayer and supplication to at least 3 other entities in addition to God. By this definition it is polytheistic.

You will not find a single decent argument to refute this. Christian apologists try as they may cannot explain this glaring contradiction.

All other beings or entities mentioned in the scriptures are not Gods or demi gods, they are part of the creation.

Man in attempt to satisfy his own needs and fancies has created and creates other gods or demi gods.

Worship and supplication to Christ as Lord and/or God and worship and supplication to Mary are examples of polytheism.

I feel that it should also be noted that there have been questions regarding the unity of Jesus with God the Father.

Most of the time, these non-trinitarians simply disagree upon the divinity (or more accurately, the deity) of Jesus. However, some actually believe in the deity of Jesus as a separate, but lesser god.

One particular faith that believes this is the Jehovah's Witness belief system.

Having said all of this, the majority of Christianity believes in one, single God. It actually comes from many passages throughout the bible. Here's one I like:

Deuteronomy 6:4 (NIV)

Hear, O Israel: The LORD our God, the LORD is one

(Other translations read "The LORD is our God, the LORD alone".)

Pretty solid backing for monotheism.

Christians consider the three-in-one Trinity to be one God. It can't claim to fulfill Judaism if it viewed the Trinity as more than one God. Once that line is crossed it would be openly denying a fundamental part of its very own roots. So it won't happen. God says numerous times that he's one. There are no gods before him, or after, above or below him (Deuteronomy 4:39).

So when this takes effect at Mt. Sinai it's obvious that only the god that was introduced there is God. Any other gods, or forms of God, that are not introduced at Sinai to the Jews can't claim to be God. Because he established himself that this god is it, there are no others. And then of course the question is why don't Jews accept the Trinity. The answer is Jews were not introduced to a Trinity by God at Mt. Sinai. Jews were only introduced to God (The Father), who said he is one.

As for the supernatural characters you describe, God says that even false prophets can do miracles (Deuteronomy 13:1-10). So supernatural abilities don't prove anyone as being a god and certainly doesn't mean they should be worshipped.

Ray has given the first part answer to the question, but OP is waiting for the second part.

This is a really great answer. Unfortunately I phrased my question as two parts and the responses here are split on what they answer: you addressed the "other gods are false" part, but quite perfectly. Would you care to offer some thoughts on the second part, that is, given that you acknowledge the existence of Satan and other beings like angels within the bible, on what basis is God a deity and those are not? (Others have said "because He created them" but there must be more to it, since other religions/mythologies have gods creating other gods too.)

I'm now attempting the second part of the answer. Andrew Vit (OP) gave these as examples as other beings to be explained:

The Biblical universe also includes a varied cast of supernatural characters like Lucifer, Metatron, Archangels, Demons, Seraphim, etc. Some of these manifest with power in the world and represent the roles of messengers, warriors, healers, advisers, protectors, troublemakers, and so on.

As Ray implied in his answer, monotheism is OUR terminology, not the Scripture's terminology. Based on the revelatory data of the Bible, it is WE as modern readers who try to construe the nature of this one deity as well as the nature of the other beings listed above. So the answer to "on what basis is God a deity and those are not" is a matter of OUR definition, not the Bible's. For example, what is the nature of Satan? There is nowhere in the Bible that the nature of Satan is described systematically, since the Bible is not a theology book. There are only sporadic references to Satan such as Genesis (the serpent), Job 1:7, Isaiah 14:2 (morning star), Zechariah 3:1-2 (in a vision), Matthew 4:1 (devil tempting Jesus), Luke 10:18 (Jesus saw Satan fell from heaven), Revelation 12:7-12 (Satan thrown from Heaven), Rev 20:2 (Satan bound for 1000 years), etc. Therefore it is a question of philosophy and theology.

It is a legitimate question, however, but it falls on the theologians / philosophers to come up with a theory that describes:

1. the ontological nature of the being (what is it?),
2. the etiology of the being (where does it come from? how does it come into being?), and
3. the purpose/function of the being (what does it do in the scheme of things? what are its powers?)

Which theology? Andrew specified Christianity. But Christian theology wasn't static. We also need to include new development during the Intertestamental period (aka Second Temple period) covering the period between Malachi and Jesus where a lot of those beings started to populate the apocryphal, pseudepigraphical, and apocalyptic Jewish writings in that period, including the Dead Sea scrolls. Some Greek myths and philosophical influence also need to be considered. New Testament period theology as expressed in the NT books take all those as the background.

So my answer is going to be the mature Christian theology around 11th century. By that time, Christian understanding of God has included following attributes: omniscient (infinite knowledge), omnipotent (unlimited power), omnipresent (present everywhere), transcendent (outside space and time), eternal, etc. Does this being exist? Yes, says St. Anselm, using his famous ontological argument from 1078 which has YET to be conclusively defeated. Because there can only be ONE of this kind of being, then all other beings by definition can only be LESS than God. This then answers your question "On what basis is Christianity classified as monotheistic given the existence of other powerful beings in the Bible". It is monotheistic because there can only be one being that has those attributes (per Anselm's ontological proof).

About the other beings about whom you ask "Why are these not acknowledged as 'gods' or 'lesser gods' like in the pantheons of other religions: how is the nature of their existence different?", the answer is simply: because according to mature Christian theology the only deity worth its salt can only be one. It's a matter of terminology, so Christian theology DOES make a place for angels and demons, but Christian theology would NOT call them gods like Greek mythology, for example, because a Greek god doesn't have all attributes necessary to fit St. Anselm's God. There exists this one being God, said St. Anselm, so logically it follows that all other beings don't fit to be called god.

What are these lesser beings' nature? I would refer you to St. Thomas Aquinas's writings if you accept his theological, philosophical, and scripture interpretation presuppositions. Or other readers may suggest other systems with different presuppositions.

This subject is largely what separates Latter-Day Saints (LDS) from other Christians. We believe God the Father to be the literal father of our spirits, and Jesus Christ (God the Son) is actually one of our spiritual siblings.

Romans 8:16-17; (KJV)

16 The Spirit itself beareth witness with our spirit, that we are the children of God:

17 And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.

When God created us in his likeness and image, he did so in more than just appearance. Families exist on Earth much like they do in Heaven. And just as a father on Earth wants to see his children grow up and inherit all that he has created for them, our father in heaven wants us to return to him and be worthy to inherit his kingdom, where we can become Gods like him, with Christ.

Such a gift is only made possible through Christ, he is our mediator with the father, he suffered the atonement to satisfy the demands of justice and mercy, it is through Christ's mercy alone that we can return to live with our father. The God of the old testament was Jehovah, who in the flesh was called Jesus Christ, and it is only through him that we can receive eternal life. Hence the commandment, "Thou shalt have no other Gods before me."

We lived before the creation of the world as spirits, we are all siblings, sons and daughters of one father, our father has a father, and his father has a father. Just as it is in mortality, it is the same in immortality. How many Gods exist within the eternal realm is not known to us, but no doubt they are innumerable. What we know of them is virtually nothing, it is not requisite knowledge for us to accomplish the purposes of Earth. We have one objective while we are here: Follow Christ, and only Christ, he is the key to our salvation, and exaltation.

I find the Bible to display a bit of a broader perspective--or, perhaps, several different perspectives--than a simple term like monotheism.

In one sense, as others have argued, the Bible makes clear that there really is only one God. Belief in other "gods" is a joke, because those would-be gods are made by the very people who worship them. This is the perspective in, for example, Jeremiah 5:7: "Your children have forsaken me and have sworn by those who are no gods." (ESV) In this sense, the Bible displays a fundamentally monotheistic perspective.

On the other hand, the Bible talks about other "gods" in more than one way. For example, I found 9 cases in which God either warns his people not to worship other gods, or is angry at them for doing so. In these cases, they are spoken of not as idols, but as gods. Likewise, there are over 100 references to "my God", without denying that "my God" is the only god anyway.

Or see Joshua 24:15 (ESV):

And if it is evil in your eyes to serve the Lord, choose this day whom you will serve, whether the gods your fathers served in the region beyond the River, or the gods of the Amorites in whose land you dwell. But as for me and my house, we will serve the Lord.”

Don't hear me to be arguing that the Bible is claiming that these gods are the real deal, or that scripture is advocating henotheism--in none of these cases is the Bible claiming that other "gods" truly are gods. However, the perspective that seems apparent is that inasmuch as they are worshipped as such, they are de facto gods to the people.

So, I would say that the Bible has a broader perspective than you might imagine. Ontologically, scripture is clear that God is the only God, and Christianity (and Judaism) is thus monotheistic. However, the perspective of some passages allows that, functionally speaking, men do worship many gods.

Christianity is monotheistic in that it acknowledges only one spiritual entity as God. It then goes further to say that this is because there is only one Divinity, i.e. that the condition for someone or thing to be worthy of being worshiped and acknowledged as God includes the condition of singularity, inasmuch as things such as 'omnipotence' in any true sense must belong properly to only one. Thus divinity in the Christian conception presupposes a monotheism, instead of arriving at it from selecting a god among gods to be 'the true god'.

The history of the faith through its Jewish roots suggests that men came to understand the God among gods first, and then later began to understand that his divinity was singular and complete and absolute.

The other spiritual entities, being separate from the divinity in nature, cannot therefore be the God, even if they have been worshiped in other places and times as gods by other human beings.

The Ten Commandments were originally written for a band of desert wanderers who had recently escaped a polytheistic culture. They apparently brought some of the polytheistic beliefs with them, because even while Moses was up on the mountain, they were building an idol:

[Aaron] took the gold from them, formed it in a mold, and cast an image of a calf; and they said, "These are your gods, O Israel, who brought you up out of the land of Egypt!" —Exodus 32:4

The subsequent history of Israel shows that polytheism never fully died out, and was practiced even by kings:

For Solomon followed Astarte the goddess of the Sidonians, and Milcom the abomination of the Ammonites. —1 Kings 11:5

[Ahab] took as his wife Jezebel daughter of King Ethbaal of the Sidonians, and went and served Baal, and worshiped him. He erected an altar for Baal in the house of Baal, which he built in Samaria. Ahab also made a sacred pole. Ahab did more to provoke the anger of the Lord, the God of Israel, than had all the kings of Israel who were before him. —1 Kings 16:31b-33

For [Manasseh] rebuilt the high places that his father Hezekiah had destroyed; he erected altars for Baal, made a sacred pole, as King Ahab of Israel had done, worshiped all the host of heaven, and served them. —2 Kings 21:3

The prophets make it clear that these idols are not beings like God:

Their land is filled with idols; they bow down to the work of their hands, to what their own fingers have made. —Isaiah 2:8

Everyone is stupid and without knowledge; goldsmiths are all put to shame by their idols; for their images are false, and there is no breath in them. They are worthless, a work of delusion; at the time of their punishment they shall perish. Not like these is the Lord, the portion of Jacob, for he is the one who formed all things, and Israel is the tribe of his inheritance; the Lord of hosts is his name. —Jeremiah 51:17-19

So the "no other gods before me" commandment is not an acknowledgement of the existence of other deities, but of the existence of other objects of worship.

Disciples of Christ believe in the existence of (powerful) invisible created beings such as those mentioned above. But none of them are worshiped; worship (in Greek, latreia) is only to be given to the (uncreated) Lord God of Abraham, Isaac, and Jacob. In that sense, Christianity is monotheistic.

Your question surprises me; the real issue of the monotheism of Christianity for me doesn't lie with the existence of the unseen realm, but instead with the notion of the Trinity as One God.

Christianity is monotheistic as its adherents believe in only one God.

The first commandment speaks about the gods of other religions. Although Christians believe in only God, they do acknowledge that other gods are believed in by non-Christians.

In other words, there are many gods in which to believe, but you believe in only one if you are Christian.

Also, the same passage applies to Muslims and Jews.

To answer the third part of your question: the belief in the Christian God (i.e. the Trinity) is what defines Christians.

Belief (or lack thereof) in the supernatural attributes of Satan, the Saints, Virgin Mary, angels vary by interpretation, but in general these figures are considered non-divine, and certainly subsidiary to God.