A surprise sexual act. It's a bit tricky to do, and you can only get away with it once, so follow it carefully the first time:
1) During sex, pull out and ejaculate into your hand (if you've ever spiderman'd someone, the first step is the same)
2) Ask your partner if she wants "to hear angels speak."
3) Suck the semen into your mouth while she answers.
4) Spit into her ear.
It's based on the belief that the voices of angels had life-giving power to any who could hear them, so if she gets pissed, just explain that it was a religious lesson.
A similar act is "the divine conception" when you spit into a virgin's vagina instead of her ear, and "The Word Of God" if you do it while having a giant bushy beard.
"Bro, my girl got mad pissed when I Angel Speak'd her last night, but chilled the fuck out when I explained it was just a theology lesson."