False teachers had ‘wormed their way’ into the church, with claims that God’s grace meant they could live immoral lives. They weren’t to be believed. They were a danger:
Jude 1:12-13 When these people eat with you in your fellowship meals commemorating the Lord’s love, they are like dangerous reefs that can shipwreck you. They are like shameless shepherds who care only for themselves. They are like clouds blowing over the land without giving any rain. They are like trees in autumn that are doubly dead, for they bear no fruit and have been pulled up by the roots. They are like wild waves of the sea, churning up the foam of their shameful deeds. They are like wandering stars, doomed forever to blackest darkness.
As Jesus had said, deceptive teaching is like yeast. You don’t need much to affect everything your believe.