The Bible speaks about sin all throughout from Genesis to Revelation. Sin is transgression of God's Law whether by word, thought or deed. All sin opposed to God. To sin is to hate God. The wages of sin is death or Eternal Punishment in Hell (Romans 6:23). God must punish sin because He is just. It is just like how a just judge in a court of civil law cannot allow a murderer to go free because he is a just good judge. God hates sin
No one who is unrighteous shall enter the Kingdom of God (1 Corinthians 6:9). This is because God cannot permit unrighteousness. All unrighteousness, no matter how small it may seem compared to other acts of unrighteousness is still unrighteousness.
Despite that one cannot be saved by works, as the works of the unrighteous are like filthy rags to the Lord (Isaiah 64:6), the one who is truly saved produces good fruit. Every tree that does not bear good fruit is cut down and is thrown into the fire" to be destroyed (Matthew 7:19). Good works are evidence that one is saved. This is not to say that a person who is truly saved does not sin at all, but rather that the one who is truly saved should be performing good works as faith without good works is dead (James 2:17).
Jesus said that if one loves Him, one will keep His Commandments (John 14:15). Jesus came not to abolish God's Law but to fulfil it (Matthew 5:17-18). God's Law will never change. Thus, if something in the Bible is not said to be no longer part of the Covenant, one should never dismiss it as merely a part of the Old Covenant that no longer applies. The classic example is that of usury. Many Christians argue that because it was given to the Israelites under the Old Covenant, it is no longer relevant. This really angers me because it is to say that the prohibition on usury by God is only a matter of custom or sacrifice. The Law against usury is relating to the heart, not sacrifices. Proverbs 28:8 tells us that usury is unjust gain and oppression of the poor: "He that by usury and unjust gain increaseth his substance, he shall gather it for him that will pity the poor".