Well if someone believes in false doctrine, whether knowingly or unknowingly, they aren’t believing in what God says because the holy bible is the only source of God’s pure and holy word in written form. If they don’t believe in the word of God, then they’re believing in what man teaches. The ones knowingly believing false doctrine need to repent or be damned to hell, if there is a chance for them. The ones unknowingly believing in false doctrine are most likely following false teachers or just being ignorant and not understanding the truth, but they need to turn to bible doctrine. They need to believe God rather than men. I’m not saying there aren’t good teachers who teach from the bible. There are indeed teachers, preachers etc. who are called by God and filled with the Spirit to teach His word, not the words of men