Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.

Galatians 5:19–21

In his letter to the Galatians, Paul clearly lists what is considered sinful. Sin can be thoughts, actions or attitudes that go against God’s will. Sin is, ultimately, anything that keeps us from God and will keep us from being with Him.

Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned: (for until the law sin was in the world: but sin is not imputed when there is no law. Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam’s transgression, who is the figure of him that was to come. But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many.

Romans 5:12-15

Paul tells us here that sin entered into the world through one man; Adam represents humankind in this example. Through Adam’s transgression, mortality was introduced. Paul goes on to explain that the law did not introduced sin, but identified what sin is. However, Jesus offers us a way out of spiritual death through the free gift of grace.

John goes on to tell us that sin is lawlessness and that Jesus came to take away our sins (1 John 3:4-5). If we accept Christ and follow His commandments, then we abide in Him, and those who abide in Christ, do not sin (1 John 3:6). Therefore we must put away sin and follow Jesus to avoid the spiritual death that Adam introduced.