1 Corinthians 6

12 “All things are lawful for me,” but not all things are helpful. “All things are lawful for me,” but I will not be enslaved by anything."

When all things are lawful then how is it possible to sin?

The law was fulfilled by Christ. All sin has already been punished. Christ took all the punishment that the law demanded for us.

Sin and all that stuff that is "right and wrong" was all dealt with on the cross.

It is not a matter of good and bad. What does matter is that you believe in the One that completed all the work that the law demanded.

To fellowship with God and be in his presence you must be perfect as God is perfect. God is a just God and cannot abide sin in His presence. The law is perfect as God is perfect. The law came from God but the law is not God. In order to fulfill law that is perfect you must also be perfect. We are not perfect. This is why we do not place our faith in the law but into the One that was perfect and fulfilled all of the law on our behalf.