It is not pretty. The Bible has no sympathy for false teachers.
Jesus warned against false prophets, likening them to ferocious wolves in sheep’s clothing (Matthew 7:15). Clearly they look benign but are actually dangerous.
The early part of the Sermon on the Mount was dedicated to correcting false teaching: “You have heard that it is said… but I tell you…” (Matthew 5:17-48).
Matthew 23 is a lengthy denunciation of the Pharisees and teachers of the Law. Jesus condemned their hypocrisy and their false teaching (e.g. vv.16-22). He called them hypocrites, children of hell, blind guides, blind fools, snakes and a brood of vipers, and murderers.
Jude’s entire letter (only 25 verses) is about “contending for the faith” in the face of false teachers. They are said to “pervert the grace of God into a licence for immorality” (v.4). Jude lists many adjectives, calling these people “ungodly… blemishes… shepherds who feed only themselves… clouds without rain… autumn trees, without fruit and uprooted – twice dead.. wild waves of the sea, foaming up their shame… wandering stars… grumblers and fault-finders; they follow their own evil desires; they boast about themselves and flatter others for their own advantage.” They follow mere natural instincts and do not have the Spirit (v.19).
Most significantly, Jude emphasises their demise. Their condemnation was written about long ago (v.4). They are subject to God’s “Woe to them!” (v.11) and linked to Cain and Balaam (v.11). They are said to have been destroyed in Korah’s rebellion (v.11). They are reserved for blackest darkness forever (v.13). Moreover, Jude points to previous judgements that “serve as an example of those who suffer the punishment of eternal fire” (v.7) and he quotes Enoch as saying:
Jude 14-15
14 Enoch, the seventh from Adam, prophesied about them: ‘See, the Lord is coming with thousands upon thousands of his holy ones 15 to judge everyone, and to convict all of them of all the ungodly acts they have committed in their ungodliness, and of all the defiant words ungodly sinners have spoken against him.’
We could go on. There are many passages about false teachers, but that should be sufficient.