How should a Christian evaluate whether a particular action a sin or not?

Whether or not it tends towards doing the Christ's work of saving humankind.

Thus, employing others for a wage, the use of violence for any reason, making claims of private property or unequal right to wealth, spreading false rumors and hatred, etc., are all sinful; while consensual sexual relations of any sort among adults, passing on knowledge or working to create new knowledge, playing basketball, etc. are not sinful.

How should a Christian evaluate whether a particular action a sin or not? To determine this we must find the real meaning of sin.

Sin can now be defined as: Any mistake or error[1], action or non-action that can cause a persons own attitude to enter into a state of anxiety or worry[2], a lack of faith[3], to feel guilt[4], anger[5], depression[6], fulfilling the desire of the flesh over the desire of the truth[7], to cause retribution through the fulfilling of the law[8], or any from of idolatry[9].

[1] Any mistake or error.

Therefore, to him who knows to do good and does not do it, to him it is sin. (James 4:17)

We are of God. He who knows God hears us; he who is not of God does not hear us. By this we know the spirit of truth and the spirit of error. (1 John 4:6 NKJV)

This is a double teaching for it shows also how to test the spirit. By if one can hear your words. Also we can see that what is not of truth is error.

[2] Anxiety and Worry

Therefore I say to you, do not worry about your life, what you will eat or what you will drink; nor about your body, what you will put on. Is not life more than food and the body more than clothing? (Matt 6:25 NKJV)

With a little research you can find out how mental illness is connected to anxiety. this will also help you make the connection between healing and faith.

[3] Lack of faith

But he who doubts is condemned if he eats, because he does not eat from faith; for whatever is not from faith is sin. (Romans 14:23 NKJV)

[4] Guilt

let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience and our bodies washed with pure water. (Hebrews 10:22 NKJV)

Guilt is a sign of a lack of faith.

[5] Anger

Let all bitterness, wrath, anger, clamor, and evil speaking be put away from you, with all malice. (Ephesians 4:31 NKJV)

There is a teaching saying that righteous anger is ok because Jesus got angry. The point that is missed is that he is the only one who is allowed to get angry.

for the wrath of man does not produce the righteousness of God. (James 1:20 NKJV)

Beloved, do not avenge yourselves, but rather give place to wrath; for it is written, “Vengeance is Mine, I will repay,” says the Lord. (Romans 12:19 NKJV)

[6] Depression

Combined with analogies for depression is also analogized anger as "Weeping and Gnashing of Teeth". I find this interesting that it points directly to the sons of the kingdom. It takes a good bit of explaining to describe the analogy for darkness, but to summarize poorly and from a certain point of view, it means to not completely have your focus on God's word.

But the sons of the kingdom will be cast out into outer darkness. There will be weeping and gnashing of teeth. (Matthew 8:12 NKJV)

In usage you can see how the fires of angry truth result here.

If anyone does not abide in Me, he is cast out as a branch and is withered; and they gather them and throw them into the fire, and they are burned. (John 15:6 NKJV)

Notice the depression involved from being cut away from God's vine?

[7] Fulfilling the desire of the flesh over the desire of the truth.

For what the law could not do in that it was weak through the flesh, God did by sending His own Son in the likeness of sinful flesh, on account of sin: He condemned sin in the flesh (Romans 8:3 NKJV)

God is about attention, the pure attention to the truth. Since the body feel pain and discomfort, when we choose to pay attention to need of the body not perceiving the entire truth about entire all over affect that the action will cause. This is sin. For in not seeing the entire truth, attention to the truth has been given to the creation (in this case our own bodies) instead of the creator which is Idolatry.

[8] To cause retribution through the fulfilling of the law.

For God to keep his word the law must continue to be fulfilled, yet we are no longer under this duty for in our error in keeping to his word. He has decided to make himself true.

I looked, but there was no one to help, And I wondered That there was no one to uphold; Therefore My own arm brought salvation for Me; And My own fury, it sustained Me.(Isaiah 63:5 NKJV)

Do not think that I came to destroy the Law or the Prophets. I did not come to destroy but to fulfill. (Matthew 5:17 NKJV)

Beloved, do not avenge yourselves, but rather give place to wrath; for it is written, “Vengeance is Mine, I will repay,” says the Lord. (Romans 12:19 NKJV)

He who leads into captivity shall go into captivity; he who kills with the sword must be killed with the sword. Here is the patience and the faith of the saints. (Revelation 13:10 NKJV)

But I will show you whom you should fear: Fear Him who, after He has killed, has power to cast into hell; yes, I say to you, fear Him! (Luke 12:5 NKJV)

[9] Any from of idolatry.

Therefore put to death your members which are on the earth: fornication, uncleanness, passion, evil desire, and covetousness, which is idolatry. (Colossians 3:5 NKJV)

The easiest way to understand idolatry is "Anything that your focus is on that separates you from the person talking to you." If the TV stands in front of the word, then the TV is the idol. If the laptop stands in front of the word, then the laptop is the idol, if the phone texting is going on in front of the word, then it is the phone that is the idol. Explained though St. John this way:

Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him. (1 John 2:15 NKJV)

Therefore since anything can be an idol, even that which is on your mind. Know that God hates this one so very much. As you yourself may have even expressed anger at IT standing in front of what YOU say.

Since they are directly addressed in the Ten Commandments, most people agree that committing adultery or murder is sin Yes, even in the teachings that Jesus gave we find that he still considers disobedience to the ten commandments sin.

You know the commandments: ‘Do not commit adultery,’ ‘Do not murder,’ ‘Do not steal,’ ‘Do not bear false witness,’ ‘Do not defraud,’ ‘Honor your father and your mother.’” (Mark 10:19 NKJV)

Yet this world is a complicated place full of things that are not so clearly addressed. Some traditions believe using electricity or wearing pants are sins, others do not go so far. The complication comes because not only do we ourselves not want to sin, we also do not want to be a cause for others to sin.

Sin separates you from God. So to explain what this means. Your perception of the truth around you is how well you are communicating with God. As you sin you become oblivious to the reality you are in. Idolatry is the main root of sin. For through the knowledge of Good and Evil Idolatry was born, for in the absence of the knowledge of Evil, judgement did not exist. For by declaring a thing to be evil we show partiality to a part of God since God is in all things. For if this thing is good, and this ting is bad, then by simple logic we have declared God bad. Faith then undoes the damage of that knowledge, for now we can know, but we can also trust as in early days of Adam.

What guidelines can a Christian use to evaluate whether a specific action is sinful or not? You can answer these questions to yourself in all you do. Does this action cause myself anxiety, anger, depression? If so, What do I not believe? If you found that you have made an error and you worry about the consequences Where is your belief in humility, and blessing those the curse you? If you find yourself filled with focus to fulfill the desires of your flesh, know that you are filled with sexual anxiety. This will lead you to death. Fill your mind instead on good things, or empty your mind completely. If you feel the need to get even, forgive and you'll be forgiven otherwise face the penalty of being under the law. If you find yourself loving one part of God over another part of God you have given favoritism, so that puts you under the law of death. If you do not love what is being told to you, you will not enter into the kingdom of the heart. If you can not love what is being told to you more then anyone that you love, you will not enter into the kingdom of the heart.

Failure to fulfill the ten commandments and have faith, you will react with storming outside angry and depressed. And you will be separated from your loved one.

Sin does require full knowledge that the particular action is sinful.

Whenever evaluating something as sinful you must take into account the

1. Circumstances:
   1. Killing out of self defense
   2. Righteous Anger (not self directed, but in protection of others)
   3. Had no way of grasping the moral/ethical reality. (this could be said of lots of cohabitating couples today)

2. Object:

   What is the thing being acted on. These are the 'Ends'. If one does harm to a completely innocent creature, then you're committing a 'graver' sin than if you do something to a mostly innocent creature. (Difference between exterminating rats in your bas*m*nt and hitting them with baseball bats for fun)

3. Intention:

   Ends cannot justify the means, but the intention is important. If you start out with bad intentions, it really doesn't matter what good you do, although hopefully you'll learn that doing good is much better than doing evil!

This is what I taught to my 8th and 9th graders last year in Religious Ed. it comes out of a faith formation handbook for teenagers from St. Mary's Press.

Sin is lawlessness

God has given us commands based upon His nature. Meaning that the right thing to do is based upon what He would do and the wrong thing to do is against what God would do.

1 John 3:4 NKJV

Whoever commits sin also commits lawlessness, and sin is lawlessness.

The new testament puts a spin on law, instead of us following the law, we are now to follow exactly by Jesus examples. Basically, living by faith has replaced the need to live by the law. So what is sin today?

Romans 14:23 NKJV

But he who doubts is condemned if he eats, because he does not eat from faith; for whatever is not from faith is sin.

Whatever is not from faith is sin.

Instead of looking to the law we now look to Jesus. But remember, the law is based off of Jesus nature and Jesus followed the law to the letter. So sin is still to not follow the law, but it is also to not follow Jesus.

Sin as a condition is separation from God. A sin as an action is an action that brings about such separation from God; such an action "misses the mark," which is the Greek word for the act of sinning. This is the general---very general---rule of thumb. The trick is to tell what separates us from God.

A good rubric, for those who already know in their heart of hearts what is right and wrong, is that given by Mississippi comedian Jerry Clower in his album Classic Clower Power, disc 2, track 25 "You're Fixin' to Mess Up" (cf. http://jacksonianlawyer.wordpress.com/2009/07/25/some-sage-words-of-advice-from-the-legendary-jerry-clower/). For the sake of those yet unable to endure heavy Southern accents, here is the rubric distilled:

If you’re fixing to make a decision about what’s right and what’s wrong in your life, do you ask other people’s opinion about it? That’s a pretty good indication you're fixing to mess up.

Number two: do you argue with yourself? ...if you’re arguing with yourself, [that's a] pretty good indication you should not do it.

Number three: do you feel uneasy when you do it?

Number four: Can you give thanks and say ‘Lord, I thank You for providing this for me.’? The Bible says, give thanks for all things. So when you do it, can you say ‘Lord, thank You for providing this for me. [Indeed, I thank You] for [making things the way they are, so that] I can commit to what it is I’m doing.'?

What is right or wrong? Do you ask other people? Do you argue with yourself? Do you feel uneasy when you do it? Can you give thanks and say ‘Lord, I thank You for providing this for me.’? If you can’t, you [had] better watch out…you’re fixin’ to mess up.

N.B. in Southern dialect, to be fixing to do something is to be about to do something.

At the root of sin is self-will and insistence upon our own way at the expense of submitting ourselves to our Father in heaven Who knows better. The resulting cognitive dissonance brings about the signs 1--3 above.

Edit: as @Jürgen A. Erhard has pointed out, this rubric is insufficient on several counts. In my opinion, this is because it displays the signs of cognitive dissonance, which (thank God) accompany a struggle with temptation. These signs are useful in a sort of basic triage to call oneself to attention: if experiencing the signs in this triage (or quadriage), mustering all your dispassion, immediately ask yourself, "Have I committed to doing something wrong, and am I now only trying to justify myself and my wrong actions?"

However, cognitive dissonance may also accompany any difficult decision, which is not necessarily a result of wrongdoing. Also, weighing both sides of a difficult decision is, presumably, an attempt at wisdom and not an attempt at self-justification; and furthermore, asking someone else (whom you respect as someone following the truth, not just someone whom you know will agree with you) is a manifestation of humility. So perhaps at the root of Clower's observation is that one must flee all appearance of sin: in particular, the self-justification that accompanies an assent to sin.

This is a way to tell if you are doing wrong. It is another story to tell what is right, i.e. what is best.