This question is simple and I am interested in what people have to say, but first let me just define sin based on how John put it.
1 John 3
4Everyone who practices sin practices lawlessness as well. Indeed,
sin is lawlessness.
So sin is basically a transgression of a law. There are numerous references to laws in the Bible. There is the law of Moses, the law of God, the law of Christ, the law of freedom, etc.
In the New Testament, there are laws still given to people that were continued to be taught, such as the 10 Commandments. They can be found here and there throughout the New Testament, and Jesus even listed some in Matthew 19, saying they are necessary to keep in order to enter eternal life.
So we know that sin is a violation of something that has been codified into law, i.e., God's law.
With all of this in mind, why would someone not enter the kingdom of God or be cast into the lake of fire for doing something that wasn't codified as a law and, arguably, isn't actually a sin?
Revelation 21:8 says cowards and sorcerers, get cast into the lake of fire. Is there is a law against being a sorcerer or coward?
Galatians 5:19-21 mentions that those who are impure, practice debauchery, sorcery, hatred, discord, jealousy, rage, drunkenness, etc will not enter the kingdom of God. While I fully know and agree that these things are unsavory and bad, where were they codified as sin?
Is the conclusion here that people can miss out on eternal life without actually being guilty of sin, but rather something that is immoral, distasteful, or otherwise frowned upon?