Sin is the transgression of God's law. To break what God has told you to do or not to do is sin. " 1 John 3:4 Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law. " God's law has not always been the same. There was a law in the garden. There was a law called the law of Moses for the Jews. Today there is the Law of Christ," Gal. 6:2 Bear ye one another's burdens, and so fulfil the law of Christ.", the Law of Faith," Rom. 3:27 Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith.", or the Law of Liberty, " James 1:25 But whoso looketh into the perfect law of liberty, and continueth therein, he being not a forgetful hearer, but a doer of the work, this man shall be blessed in his deed. ". In spite of what people may say there is a law today for us. Many misunderstand the Bible because much of what it is talking about when it simply says "the law" or "law" pertains to the law of Moses. Because of this misunderstanding many proclaim that we are not under any law today. But we are or else we could not sin.
In the garden it was not sin for Adam and Eve to be naked. " Gen. 2:15-17 And the LORD God took the man, and put him into the garden of Eden to dress it and to keep it. And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat: But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die. " In the garden they had a positive command and a negative command. They had something to do, dress and keep the garden. They had something not to do, not eat of the tree in the mist of the garden. So in the garden it was not sin for them to go naked, but today it would be a sin. (1Tim. 2:9)
Under the Law of Moses it was not sin for a man to have more than one wife. In fact many had more than one wife. David had many wives. This was not sin because it did not transgress God's law for them. However today for me have more than one wife would be sin indeed. For Paul wrote " 1Cor. 7:2 Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband. " Today we are to have one wife for one man.
You see sin has not nor is it the same today as it was in times past. " Heb 1:1-2 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds; " Today we look to Jesus Christ. We follow the teachings of his and those of his holy apostles. " 1Cor. 14:37 If any man think himself to be a prophet, or spiritual, let him acknowledge that the things that I write unto you are the commandments of the Lord. " Our law began when Jesus died on the cross. " Heb 9:15-17 And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance. For where a testament is, there must also of necessity be the death of the testator. For a testament is of force after men are dead: otherwise it is of no strength at all while the testator liveth. "