Galatians 5:19 Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality, 20 idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, 21 envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you, that those who "practice" such things will not inherit the kingdom of God. *These are not believers.
1 John 3:9 - No one who is born of God "practices" sin, because His seed abides in him; and he cannot sin, because he is born of God.
Just because this epistle is addressed to believers does not mean that everyone being discussed in the letter must be a genuine believer. If a Pastor of a church (particularly a very large church) on Sunday morning addresses the congregation with, "good morning brothers and sisters in Christ" and the message is directed at believers, does that mean that EVERYONE who attended church that Sunday morning MUST be a genuine believer? It's not hard to find "nominal" Christians mixed in with the group.
In 1 Corinthians 6:9-10, we see a similar list of sins that were mentioned in Galatians 5:19-21, and Paul goes on to describe such people who practice these sins, in contrast with those who don't. 9 Or do you not know that the UNRIGHTEOUS will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, 10 nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, will inherit the kingdom of God. 11 *Such were some of you; but you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus Christ and in the Spirit of our God.
*Notice that Paul refers to these people who will not inherit the kingdom of God as the UNRIGHTEOUS, not the righteous. *They were not genuine believers.