Which religion was the first monotheistic one?

Long before Judaism the god Ahura Mazda was worshipped in Persia. They had a number of Commandments which where very simular to the Ten Commandments, it is very likely Abraham based his religion on this older one or at least borrowed some elements

A pity question. Atenism was a sect that departed from the traditional polytheistic Egyptians but did not really catch on after the death of Amenhotep. One, as mentioned above, can make a case for its brief appearance being the first recorded monotheistic belief system. Also often overlooked is that Judaism as practiced during the first temple period was henotheistic, i.e., recognizing the existence of other deities while worshipping YHWH or El as the primary regional or ethnic deity. Judaism changed and became more monotheistic after the Mesopotamian diaspora due to the influence of Zoroastrianism under the Persians.

Let me give an input regarding Hinduism which is considered as a major polytheistic religion and one of the oldest in the history.

Hinduism is actually have another name, 'Sanathana Dharma', which is considered as the culture of India. Even though Hinduism now considered as a religion with all essence of this culture.

There are more than 33 Million Gods of Hinduism. Why Hindus worship so many gods and goddesses is a real mystery for most people. What is the role of Hinduism in a monotheistic religion question in here? It do have an important role because the ultimate message from Hinduism lead to the concept of single God or all Gods are the same. Even if there are so many Gods like Trinity Shiva,Vishnu and Brahma, the core of the Hinduism says that all these Gods are the same. Krishna,Rama, Durga, Ganesha,etc and the number is lot more in the list of Gods. But see what Vedas says,

The most important texts in Hinduism,4 veda's mahavakyas(ultimate messages) are the following

1.Rig Veda - prajñānam Parabrahma - Wisdom/consciousness is the parabrahma(highest truth, not to be confused with 'Brahma' in Trinity)

2.Atharva Veda - ayam ātmā brahma- I am this Self is parabrahma

3.Sama Veda - tat tvam asi- You are that(parabrahma)

4.Yajur Veda - aham brahmāsmi- I am that (parabrahma)

All these creators in the world is considered as the part of one ultimate truth which can be called as a 'monotheistic GOD'. According to Hinduism, all these 33 million Gods are the same. Most of these Gods are the incarnations. Like Krishna and Rama are the incarnation of Lord Vishnu. Anyone can worship any of these Gods and get to the ultimate truth.

See a related question in Hinduism.SE here.

Also Hindus does not have strict restrictions in going to church or other temples like in Christianity or Islam. The reason is that Hindus believe that all Gods(in other religions too) are the part of the one ultimate truth or God. So isn't it the monotheism? Hindus who know the message of their culture know that everything is one.

Hinduism is one of the oldest religion, so while considering the core of message of the hinduism, it can be considered as the first monotheistic religion. It is way too older than Judaism, zourastrism or any other religions mentioned in other answers.

It is actually a chronological tie between Judaism and the religion of Ismail, Abraham's eldest son.

As I had mentioned in an earlier post, Ismail and his Father Abraham either built or commissioned the building of the Kaaba in Arabia-(specifically, in the city of Mecca). The Kaaba was built as the First physical structure or House of worship dedicated to honoring, worshiping and praying to a single, abstract-(though communicative) God in world history 1000 years before the construction of The Temple of Solomon in Jerusalem. Ismail's monotheism-(which was nearly indistinguishable from his younger brother Isaac's monotheism), was short lived/ ephemeral. After Ismail's death, the vast majority of his descendants and future descendants gradually drifted away from Abrahamic Monotheism and adopted various deities that were depicted in idolatrous form. The one exception were the Hanifs, who steadfastly maintained and retained the Abraham-Ismail monotheistic legacy and tradition for 2500 years, until Muhammad's time.

However, it is Judaism that has maintained its steadfast loyalty and dedication to the honor, worship and prayer to a single, abstract-(though communicative) God, beginning with Abraham and Isaac, though furthered by Jacob, Joseph and a long line of Prophetic figures, such as Moses, Aaron, Joshua, Elijah, Ezekiel, Daniel and Isiah. Judaism isn't necessarily the oldest or first monotheistic religion in world history; though Judaism, is, by far, the longest lasting and historically consecutive monotheistic religion in world history dating back 4000 years.

Monotheism

This does depend on the definition being used for monotheism, which there appears to be some confusion about. Originally monotheism implied there was only one God, and all religions were searching for that God (possibly erring in their perception of God). In this sense, Judaism is not monotheistic, as it says there is one God, but other religions are NOT searching for God, but are worshiping false gods.

Judaism

In the sense of the belief of there only being one God, however, the oldest recorded monotheistic religion would be Judaism and its precursors. The religion describes only one God, the mention of other deities being restricted purely to idols and to poetic script.

Atenism

Some have tried to say Atenism is the oldest religion, from 1350 BC to 1320 BC. The religion, however, did acknowledge other gods, but some have excused that detail for political reasons, the king being urged to admit other gods.

Judaism never acknowledged any other gods, but is sometimes excused of this on the grounds the Jewish people often practiced multiple religions. No doubt there were worshipers of Aten who also believed in other gods, so I believe this point to be inadmissible.

Dating Judaism and Moses

You can take several possible dates for the beginning of the religion. Judaism itself is taken from Judah (יהודה), and the modern form of the religion (containing all the books of the Tanakh) would've began around 516 BC.

The date of Moses formalizing the religion is commonly stated as 1250–1200 BC. The biblical record dates it as 1450 BC, which agrees with several archaeologists' findings. Several theories would push this date back to 1650 BC, contemporary with the Bronze Age Collapse of the Middle Kingdom of Egypt and the sacking of Canaan. (Source: Patterns of Evidence: Exodus)

Ancient Precursors

Regardless of how you date Moses, Abraham predates Atenism by centuries. There is no evidence to suggest Abraham believed there were many gods, the only accounts we have of him suggesting he believed in one God. The genealogy of the bible is a good example of this, where he and others recorded a lineage back to the first man, Adam, who himself was a monotheist as described in the religion.

The fact these beliefs are contemporary back to the time of Abraham is confirmed by similar folk history, legend and religion across the world. Many people talk of a first man, of a great flood that wiped out the Earth, of eight people who survived on a boat and repopulated the world, from Polynesia to China.

This would make Judaism and its precursors the most likely candidate for the first monotheistic religion.

Akenaten

In Egypt, Pharaoh Amenhotep IV started a new monotheistic religion and renamed himself Akenaten, moving to a new capital city Armana unsullied by the normal religion. This would be about 1350 BC to 1320 BC.

When he died, his son Tutankhamun reconciled with the old regime, and the city was abandoned. This heresy and the need to wipe out its existence is one reason why King Tut's tomb was left intact so that it could be found in the 1920s.

I know Sigmund Freud wrote a book "Moses and Monotheism" trying to tie Judaism's development to Akenaten, but most seem to be unconvinced.

Zoroastrianism

Although we usually recognise Judaism as the first monotheistic faith, the title may actually go to Zoroastrianism. Zoroastrianism was established around the 6th century BCE. In a nutshell, it abandoned the previous Persian pantheon and simplified it to

"two forces Spenta Mainyu (Progressive mentality) and Angra Mainyu (Destructive Mentality) under the one God, Ahura Mazda (Illuminating Wisdom)"

Although Judaism may be older than the 6th century BCE, Judaism was not a strictly monotheistic faith until the return of the Israelites from the Babylonian exile, which happens to be around the 6th century, from Persia.

Zoroastrianism survived as the state religion of Persia for a few centuries until Alexander the Great's arrival, after which it gradually declined. Most of the remaining Zoroastrians converted to Islam when it arrived (likely simplified by the monotheistic similarities), although there's still a lively community of Zoroastrians, many in Iran (150'000 - 2 million depending on who you ask).

In an appendix to his book "When Our World Became Christian", Paul Veyne studies the extent to which the concept of monotheism applies to Judaism. His main point is that the concept of monotheism ("there is only one God") can be differentiated from monolatrism ("ye shall worship only one God") only if the idea of "a non-existent deity" can be conceptualized. That idea appeared at one time; before that invention, people did not think of gods in terms of true/fake, but rather in terms of yours/others, or stronger/weaker. Paul Veyne references the relevant Scripture texts in which the transition from monolatrism to monotheism can be seen.

Apparently, the Babylonian captivity was a turning point because it exposed the Hebrew intellectual elites to concepts which were developed in more Eastern areas, notably Zoroastrianism. Though Zoroastrianism gained the status of "state religion" in Persia only later on, the basic concepts were older (the exact date is highly disputed, but the current consensus points to "some time in the 2nd millenium BC"). The ideas had begun to percolate to neighbouring Babylonia at the time the Hebrew were there. Among these concepts was the notion that there was a Supreme Deity (Ahura Mazda), and other "gods" were really subordinates, even proxies; every prayer sent to any god was ultimately brought to the attention of Ahura Mazda. In that sense, the other gods in Zoroastrianism were already at that time beginning to be perceived as proto-angels and demons, to be respected and/or feared, but not "gods" in the same sense of Ahura Mazda.

When the Hebrew came back from Babylon, monotheism crystallized in their minds: they now understood that a god could be fake, non-existent. This contrasts with what philosophers were thinking in 5th century BC Greece: for them, the "divinity" was basically the Cosmos, the order. Conceptual gods were a-plenty, e.g. Eros (for love) or Chronos (for time). They had no notion that a concept could be non-existent: if you can think about it, then it exists, and is part of the Cosmos, thus you can worship it.

Since monotheism was a gradual innovation, there are "intermediate states" and one cannot really pinpoint an exact year in which it happened. The term henotheism has been coined to describe these intermediaries. In the case of Europe and Middle-East, it seems that true monotheism emerged with Judaism in the 6th or 5th century BC, although some definitions of monotheism can include earlier Zoroastrianism or Atenism (as described by @Semaphore). (However, it can be said that when Akhenaten mandated what was effectively monolatry, he was more concerned about the mundane power of the Amun priesthood than the alleged fakeness of the other gods.)