What metaphor in this passage seems to promise Hellfire to those who are not true followers?
Beware of false prophets, who come to you in sheep's clothing but inwardly are ravenous wolves. (2) You will know them by their fruits. Are grapes gathered from thorns, or figs from thistles? In the same way, every good tree bears good fruit, but the bad tree bears bad fruit. A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus you will know them by their fruits.
Not everyone who says to me, "Lord, Lord," will enter the kingdom of heaven, but only the one who does the will of my Father in heaven. On that day many will say to me, "Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many deeds of power in your name? (3) Then I will declare to them, "I never knew you; go away from me, you evil-doers."
(2) Aimed at teachers who fail to follow the Apostle's Doctrine in the early church.
(3) Note that not even the successful performance of miracles is enough to prove worthiness.