Therefore take heed to yourselves and to all the flock, among which the Holy Spirit has made you overseers, to shepherd the church of God which He purchased with His own blood. For I know this, that after my departure savage wolves will come in among you, not sparing the flock. Also from among yourselves men will rise up, speaking perverse things, to draw away the disciples after themselves. Therefore watch, and remember that for three years I did not cease to warn everyone night and day with tears.
Acts 20:28-31
Paul’s warning was not exclusively to and for the Ephesian church and their elders. Sadly, where there is truth, there have always been those who oppose and teach contrary to the truth.
Some teach error because they are untrained. This is why properly trained elders (and especially vocational pastors) are a must! Ignorance of sound doctrine can be averted and remedied by means of rigorous training.
Others teach error because they are false prophets. They are trained, all right, but only to be more skilled in promulgating false doctrine. The Church, every local church, and the leaders in every church must have discernment to detect false doctrine so that it can be exposed, and refuted. And if false prophets do not genuinely repent, they must be denounced, warned against, and put out.
And note that Paul warned not only about false prophets who come into the Church from without (including via books, TV, podcasts, etc.), but they will sometimes rise up from within the Church. Sadly, some preachers and teachers who seem to start well drift and actually become false prophets.
What are we to do? (1) We must all know the truth. When we know the truth well, error becomes increasingly obvious. Knowing the truth means knowing holy scriptures! (2) We must pray for discernment that only the Holy Spirit can give us. (3) We must be loving and bold when it comes to error. We do not want to shoot first and ask questions later. But we must be prepared to call out false teaching and teachers.
Important Footnote: Being discerning means knowing the difference between “differences of opinions” regarding matters that are less clear, and about which believers may disagree—and being able to know the difference between true and false doctrine regarding essential and non-negotiable truths.