Why did father Brenning have a 666 birth mark?

I hear you on this. I thought that only Damian should ever have had that 666 birthmark. Apparently though in the world of these movies important significant people and disciples around him have the 666 mark too. For example Father Brennan had it because(even though he eventually abandoned Satan and went back to Christ) at one time he was a significant follower and had a important role(witnessing and I assume actually doing something)in bringing the antichrist into the world. I am assuming that the people at some point just physically manifest the mark on them just like someone might grow a mole. In the Omen II book it actually has a scene where the sargent(at least I think it was him) that was a disciple of Damian has his mark appear on his body. And he's proud that he earned it.

The scary clown doll is hiding under my bed.

Yes that makes sense but what doesn't is the fact that he is a priest and tries to prevent Damien from living.

An apostle from Satan would detest God.

I looked up "the omen" from 1976 they had a very detailed faq from the book!

Brennan wasn't born with the birthmark. It appeared on his body the moment Satan accepted him as a follower. In the book, it explains about Father Brennan. He was born to a poor mom, who died from eating raw fish after she was too weak to forage for firewood. Brennan was orphaned, and taken in by an order of priests who literally beat the love of God in him. He was covered with scars by the time he accepted Christ and became a priest. He went on a mission to Africa, where he met an African boy and converted him as he himself has been "converted". They enjoyed a forbidden lust until they were discovered. The boy was mutilated, and Brennan barely escaped. He gradually found himself involved with a different kind of 'religious' order that enjoyed lusts and pleasures without guilt, which was what he wanted. So, he was indoctrinated into devil worship. He eventually was called upon to help arrange the rape of the jackal and of the disposal of Thorn's baby. He did not really want to kill the baby, but he did it. Then after the switch occurred and years passed, he became tormented by nightmares iin which he saw Christ who told him he was still welcome in Heaven if he repented. Brennan ignored the dreams and they eventually stopped. Then he became riddled with cancer, and suddenly decided to try for God's forgiveness after all by trying to warn Thorn.