Sin is trespassing lawful boundaries established by God.
Sin is also treating others in manner we would not want to be treated. The last half of the Ten Commandments deals with how we treat others. Sin harms others, hates others, steals from others, but righteousness (the opposite of sin) blesses others, loves others, gives to others.
Here are some eternal promises concerning the serious, deadly topic of sin:
And as it is appointed for men to die once, but after this the judgment, so Christ was offered once to bear the sins of many. To those who eagerly wait for Him He will appear a second time, apart from sin, for salvation. (Hebrews 9:27-28)
but exhort one another daily, while it is called “Today,” lest any of you be hardened through the deceitfulness of sin. (Hebrews 3:13)
Awake to righteousness, and do not sin; for some do not have the knowledge of God. I speak this to your shame. (1 Corinthians 15:34)
For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him. (2 Corinthians 5:21)
There is therefore now no condemnation to those who are in Christ Jesus, who do not walk according to the flesh, but according to the Spirit. For the law of the Spirit of life in Christ Jesus has made me free from the law of sin and death. (Romans 8:1-2)
What shall we say then? Shall we continue in sin that grace may abound? Certainly not! How shall we who died to sin live any longer in it? For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection, knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin. For he who has died has been freed from sin. (Romans 6:1-2, 5-7)
But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin. If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His word is not in us. (1 John 1:7-10)
