I have no clue to what you are trying to say. what is sin is a simple question?
Let me see if I can clue you in. You site 1 John 3:4, which is "a" definition of sin: then u ask what is sin: so then, by that definition it is merely an examination of the laws demands. And yes it is kind of simple, though a lawyer might smile at that; that is why I said to you, you have the scriptures how readest thou them. However, let me ask you: Do you not read the scripture giving other definitions for sin? I referred to Romans 14:23 and apparently you misses the other definition given there: it should have given you a clue. In the context the act that is sinful is not a law prohibited act, on the contrary the act is allowable, but the fellow, so lead by the example of the stronger brother and not of his own conviction, is never the less sinning, because it would not then be by his own faith (i.e. he did not know for himself that it was allowable by hearing the word--his week conscience was not yet fortified by scripture). Moreover, the stronger brother ought to have the attitude Paul mentions in 1 Corinthians 10:23
All things are lawful for me, but all things are not expedient: all things are lawful for me, but all things edify not. So what is the upshot of this? Any definition of sin the bible gives, since it gives more than one, is a narrower field of view than of all the definitions together.
according to the word of God sin is the breaking of the law. 1John 3v4 Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law
This is a good example of a limited (not complete) definition, for your next question is:
what law is being broken?
And that is all that matters: I am treading over no legal boundaries, violating no written ordinance, therefore it is not sin/He or they are treading over a legal boundary there for it is sin. And what is the problem with that? The pharisees viewed plucking gran as harvesting and rubbing it in the hands as threshing, so they found fault with the disciples who did such on the sabbath. They also found fault in Christ healing on the sabbath, but they had not considered the emergency of David when he fled from Saul and ate the show bread which is not lawful for any to eat but the priests only, and he gave to his men to eat as well. So what is the precise purpose for the limited definition "transgression of the law"? It is meant to rule over human trials of prohibited acts and public disclosures that must be verified by other free men. There is no authority but God's word. But the field of view becomes a bit larger when we see that,
"for whatsoever is not of faith is sin;" for with that as our definition, we can head the advice of Paul and intelligently care for our brother's state of maturity, without offense. In the new covenant as we walk with Christ, we do sin and he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. He receives us as we are and begins to perfect our faith; if we say we do not sin, we lie and do not the truth. The legalistic error is a self deception based on this limited view of sin, and has forgotten the other definition. It is important, John's first epistle is all about that. Legalism has no freedom nor mercy, and its sister error, perfectionism, has no personal conviction of sin in it at all, their eyes have a log in them, they can't see well.
Now, for the rest of your response: not putting it off as non-important, but as it has a bit more complexity in it than you might think it has, it would be better for it to have a thread all its own. But for this wider plea,
what is sin, one could site Romans 14:5
"One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind."
And does this not handle your next comment, at least, in terms of true fellowship and liberty of conscience individual saints should grant one an other until we all should come to the unity of the faith?
according to the words of Jesus he said he would be in the grave for 3 days and 3 nights
is Good friday to Easter sunday morning 3 days and 3 nights.
do we obey Jesus voice when we keep Good friday to Easter sunday?
is this sin