We have to watch our lives and our doctrines closely.
1 Timothy 4:6 Watch your life and doctrine closely. Persevere in them, because if you do, you will save both yourself and your hearers.
2 Timothy 2:17 Their teaching will spread like gangrene.
1 John 4:1 Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world.
2 Corinthians 11:13 For such people are false apostles, deceitful workers, masquerading as apostles of Christ. 14 And no wonder, for Satan himself masquerades as an angel of light. 15 It is not surprising, then, if his servants also masquerade as servants of righteousness. Their end will be what their actions deserve.
Acts 20:29 I know that after I leave, savage wolves will come in among you and will not spare the flock.
And Peter warns us---
2 Peter 2:1-3 But there were also false prophets among the people, just as there will be false teachers among you. They will secretly introduce destructive heresies, even denying the sovereign Lord who bought them—bringing swift destruction on themselves. 2 Many will follow their depraved conduct and will bring the way of truth into disrepute. 3 In their greed these teachers will exploit you with fabricated stories. Their condemnation has long been hanging over them, and their destruction has not been sleeping.