Just a guess, but according to Wikipedia, the beginning of the Omen:
Moments later Robert is informed of a plan by the hospital chaplain, Father Spiletto, to secretly adopt an orphan whose mother died giving birth to him. Robert agrees, but does not reveal to his wife that the child is not theirs. They name the child Damien.
It would seem like he’s “faking” being a the child’s father (probably could find a better gist). “Playing the role” of a father. Etc.