Sin is the spiritual thought and or physical action that is in opposition to God's command.
Mankind was given the original 10 commandments with their accompanying law of Moses to define what sin is. It was written that before the law came, sin could not be imputed because there was nothing to inform the people what defined sin.
Essentially God gave the law so that mankind could understand his ways and if they loved him they would change theirs to match his. The unfortunate part of the action was that people inferred that sin was an outward appearance of actions only so they only changed their outward actions to match God's laws and all the while they were sinning inside with no worries about their eternal life. This is part of the message that God's Son came to inform us about as we can see in the NT references below.
Matthew 6:1 "Beware of practicing your righteousness before men to be noticed by them; otherwise you have no reward with your Father who is in heaven.
Matthew 6:5 "When you pray, you are not to be like the hypocrites; for they love to stand and pray in the synagogues and on the street corners so that they may be seen by men Truly I say to you, they have their reward in full.
Matthew 6:16 "Whenever you fast, do not put on a gloomy face as the hypocrites do, for they neglect their appearance so that they will be noticed by men when they are fasting Truly I say to you, they have their reward in full.
Matthew 23:5 "But they do all their deeds to be noticed by men; for they broaden their phylacteries and lengthen the tassels of their garments.
Matthew 23:27 - "Woe unto you, scribes and Pharisees, hypocrites! for ye are like unto whited sepulchers, which indeed appear beautiful outward, but are within full of dead men's bones, and of all uncleanness."
Matthew 23:28 "So you, too, outwardly appear righteous to men, but inwardly you are full of hypocrisy and lawlessness.
Luke 16:15 And He said to them, "You are those who justify yourselves in the sight of men, but God knows your hearts; for that which is highly esteemed among men is detestable in the sight of God.
This is why works cannot gain a person eternal life however, if a person matches or does their best to match God in their being then their outward actions will by default also match with God.
So, in the old testament the prevailing error of mankind was believing that all they had to perform to gain eternal life was simply the outward appearance of holiness. In the new testament the prevailing error has changed to a much simpler one. Now the only action one needs to perform in their existence is believing in Christ and they are auto-saved.
Men have formed a tradition that they teach where no amount of effort or works on the part of the believer has any effect on attaining eternal life. You only need to accept that your flesh is sinful and that as long as you believe in Christ your soul will be saved thus, one can go about on their merry way sinning and simply ask for forgiveness every so often and POOF they are auto-saved. This tradition essentially allows that people can sin the same exact sins for their entire life without any worries of being held accountable because it is in the flesh and what can we do about sinful flesh? They simply assert that Christ will overcome their sinful flesh for them. No effort required. What a wonderful God he is for these people.
Now if simply believing is the only necessary action required to gain eternal life then this scripture makes no sense;
Matthew 7:21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. 22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? 23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
Obviously each of those who appear before Christ in the day foretold about had to be people that believed in him since they were doing what "THEY THOUGHT" he wanted them to do and they are arguing for their salvation with assertions that they were doing God's will.
And what is this iniquity the verse speaks of? If one is auto-saved simply by holding a belief in Christ then how can one do any iniquity? Didn't Christ die to remove the sin/iniquity from those who believe?
There is also another scripture that makes no sense if belief only is all that's required;
Matthew 7:13 Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat: 14 Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.
Why should the way to life be spoken of as narrow in scripture when anyone can simply do the auto-save by believing in Christ and asking for forgiveness?
Maybe a clue can be gained from this scripture;
1 Cor 9:24 Know ye not that they which run in a race run all, but one receiveth the prize? So run, that ye may obtain. 25 And every man that striveth for the mastery is temperate in all things. Now they [do it] to obtain a corruptible crown; but we an incorruptible. 26 I therefore so run, not as uncertainly; so fight I, not as one that beateth the air: 27 But I keep under my body, and bring [it] into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
Hmmm... we need to run? strive for mastery? bring my body into subjection? aren't these the performing of actions both inward and outward? but I have been told so many times that I don't have to perform any outward actions any more because the law of Moses was done away with and isn't Christ supposed to conquer my sinful flesh for me? Ah the traditions of men, gotta have'em.....
