Actually listen to what they say...what is their teaching?
The bold words below are talking about what people teach, the good or the bad. They all have something to do with the words that are said, and what Christians hear.
Test the Spirits
1 John 4:4 Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world. 2 By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, 3 and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already. 4 Little children, you are from God and have overcome them, for he who is in you is greater than he who is in the world. 5 They are from the world; therefore they speak from the world, and the world listens to them. 6 We are from God. Whoever knows God listens to us; whoever is not from God does not listen to us. By this we know the Spirit of truth and the spirit of error.
Here too, what do they say???
1 Cor 12:3 Therefore I want you to understand that no one speaking in the Spirit of God ever says “Jesus is accursed!” and no one can say “Jesus is Lord” except in the Holy Spirit.
What do they say, what do they teach, what are their words?
2 Peter 2 But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction. 2 And many will follow their sensuality, and because of them the way of truth will be blasphemed. 3 And in their greed they will exploit you with false words. Their condemnation from long ago is not idle, and their destruction is not asleep.
What do the demons teach? Doctine will prove it.
1 Tim 4:4 Now the Spirit expressly says that in later times some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons, 2 through the insincerity of liars whose consciences are seared,
And or course, the false apostles brought a different Jesus, spirit, and Gospel to Corinth, that is why it is good to be rooted in grace, and the text, same actually as in Colossians, and Galatians, and , well...all of em!