I have told you before (see John 16:1-4). The warning was needed in the first age; it will be needed in the last. The prediction was known before the ruin of Jerusalem, and doubtless preserved many from falling victims to the seducers at that period; it must be used now and till the end to preserve Christians from the errors of infidelity, false philosophy, agnosticism.
That such attacks on their faith shall be made is a proof of Christ's omniscience; that he gives here and in the next verses premonitions of danger, with counsel how to avoid it, is evidence of his love and care for his elect.