Is a person considered an apostate and sinful if a priest places his hand on their forehead and draws the cross without their prior knowledge, and what should they do about that?

May Allah forgive you and pardon you. It is not befitting for you as a Muslim to allow a priest to draw a cross on your forehead. This is the problem, not merely his supplication for you. There is no harm in a non-Muslim supplicating for a Muslim; in fact, it is permissible for a Muslim to say "Ameen" to such a supplication. However, this is provided that it does not convey an exaltation or validation of the non-Muslim's religion, and that the supplication is directed to Allah, not to His creation. As for blessing by drawing the cross, it is impermissible. A Muslim in this situation must assert his Islam and refuse to participate. We hope that this matter does not amount to apostasy, given that the questioner denied what happened and stated that it occurred suddenly and in a few seconds, which can induce bewilderment and astonishment. As for the effect of this on the questioner in terms of obsessive thoughts and nightmares, it may or may not affect him, and there is no certainty regarding this matter.