Jeremiah could not make his complaint any clearer or his case any stronger. God is against those false prophets whose spiritual delusion causes them to invent their own message apart from God's truth. Some false teachers may be sincere, but they are still wrong. They have deceived themselves into believing their messages are true. As Jeremiah points out, their messages come psychologically from within their own minds and are not from God.
Some false prophets are deliberate liars who have no intention of telling the truth.
It is the man who denies that Jesus is the Christ. There are some who do not necessarily promote false teaching so much as they outright reject the truth of God. Those who speak evil of God, Christ, the Holy Spirit, the people of God, the kingdom of God, and the attributes of God are called blasphemers. Our Lord's implication is that spiritual seduction is a very real threat even to believers. This would account for the fact that a few genuine but deceived believers may be found among the cults.