...who is Paul's warning that people who commit these sins will not go to heaven addressed to? Who does he not want to be deceived into thinking that such sinners will not go to heaven.
It's addressed to those in the church who '
live like this'. We see that more clearly in his other warning to the church about sinning:
"sexual immorality, impurity and debauchery; 20idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions 21and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God."-Galatians 5:19-20
IOW, that person is not saved. John refers to it as the practice of sin The person who practices sin in his old, unchanged, unrepentant life is simply not born again. Doesn't matter if he never was born again to begin with or stopped believing and is no longer born again, the outcome is the same--he will not inherit the kingdom of God.
"Little children, make sure no one deceives you; the one who practices righteousness is righteous, just as He is righteous; 8the one who practices sin is of the devil; for the devil has sinned from the beginning."-1 John 3:7-8
That's why the 'once saved always saved' argument is so meaningless. It doesn't matter if you were never saved to begin with or stopped being saved. Either way, the person who is living unchanged in sin is unsaved and they are going to the fiery place, not into the kingdom of God when Jesus returns. But Peter explains that we can know that we are not deceived and will receive a rich welcome into the kingdom of God by if we are growing up into and increasing in the qualities of the Spirit:
"if these qualities are yours and are increasing, they render you neither useless nor unfruitful in the true knowledge of our Lord Jesus Christ.
10...as long as you practice these things (the qualities of the Spirit-see context), you will never stumble; 11for in this way the entrance into the eternal kingdom of our Lord and Savior Jesus Christ will be abundantly supplied to you."-2 Peter 1:8,10
As long as you are growing up in your knowledge of the Lord and increasing in the qualities of the Spirit you know your entrance into the kingdom is assured and that you are not being deceived thinking that you can live in and practice sin and you will still somehow inherit the kingdom.
Don't be deceived about this, people. Make sure you're among the saved. Unchanged people are not saved people. They belong to the devil. Don't be deceived by a doctrine that says your behavior doesn't matter and doesn't prove anything.