St. Paul was addressing believers who had strayed or were in danger of straying from orthodoxy. They were either already heretics or were in danger of becoming heretics.
All Christian heretics believe in Jesus, however imperfectly. No living person fully knows or understands God and all fall short. One man's heretic is another man's saint. Do not judge them. No heretic's path to Salvation is complete or decided until the moment of his death.
If you are not a Catholic I can rightly claim that you are a heretic, but that does not entitle me to treat you in an un-Christian manner. I have been branded a heretic and insulted for my beliefs many thees on this forum by those who claim to speak for God. In the past it has brought me to anger and caused me to sin. I do not wish to cause anyone to sin against God, nor do I suspect do you. Other's beliefs do not change what I am called to do. Whether you or they know it or not, or believe it or not the "heretic" is a child of God created in His image. Only Grace and the Fruit of the Holy Spirit will bring him home.
"Keep your behavior excellent among the Gentiles, so that in the thing in which they slander you as evildoers, they may because of your good deeds, as they observe them, glorify God in the day of visitation." - 1 Peter 2:12
Sorry; but I am going to do that very thing.
I will 'judge' them by the things they say and teach; just as I am being judged for the things that I post here in FR.