Doctrine believed bears fruit; it drives deeds and actions, good or bad.
2Jn 1:10 If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting, 11 for whoever greets him takes part in his wicked works.
Greeting and welcoming in a false teacher is to participate in his wicked deeds. Wicked deeds would begin with spreading false doctrine and enslaving others to it, to the demons behind it (James 3:15-16), and to overlords and false teacher. False doctrine feeds sin. Sound doctrine believed bears fruit of righteousness.
You cannot practice righteousness with unrighteousness nor can you practice unrighteousness with false doctrine. You cannot walk in truth while not believing & abiding in it (2John).
Consider the spirit behind doctrine, words, preaching, teaching. There's but two options: that of the biblical Christ Jesus or demons.
2Co 11:3 But I am afraid that as the serpent deceived Eve by his cunning, your thoughts will be led astray from a sincere and pure devotion to Christ. 4 For if someone comes and proclaims another Jesus than the one we proclaimed, or if you receive a different spirit from the one you received, or if you accept a different gospel from the one you accepted, you put up with it readily enough.
2Co 11:20 For you bear it if someone makes slaves of you, or devours you, or takes advantage of you, or puts on airs, or strikes you in the face.
Gal 1:6 I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel— 7 not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. 8 But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. 9 As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed. 10 For am I now seeking the approval of man, or of God? Or am I trying to please man? If I were still trying to please man, I would not be a servant of Christ.
1Ti 6:3 If anyone teaches a different doctrine and does not agree with the sound words of our Lord Jesus Christ and the teaching that accords with godliness, 4 he is puffed up with conceit and understands nothing. He has an unhealthy craving for controversy and for quarrels about words, which produce envy, dissension, slander, evil suspicions, 5 and constant friction among people who are depraved in mind and deprived of the truth, imagining that godliness is a means of gain.
1Ti 4:15 Practice these things, immerse yourself in them, so that all may see your progress.16 Keep a close watch on yourself and on the teaching. Persist in this, for by so doing you will save both yourself and your hearers.
1Jn 3:9 No one born of God makes a practice of sinning, for God's seed abides in him; and he cannot keep on sinning, because he has been born of God. 10 By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother.