i think this is what happened:
- Gabriel and Marie conceive a child
- Gabriel gets sent on a mission that lasts, um, more than 9 months
- The Order looks at one of their prophesies and go, oh dear, this Gabriel guy is trouble. We'd better take Marie's child and not tell Gabriel about him. Especially, cause, he's prophesied to save us from him and stuff. They explain everything to Marie.
- Gabriel comes home and they happily reunite. Gabe then kills Marie while wearing the Mask (wonder if when he was possessed by the mask he knew the trouble his offspring would give him, so he decides to kill Marie to be sure that he's never troubled by it?), but she's okay with it. The order might have even sent her on a mission to speak with the founders of the order, and she had to be dead in order to do that anyways. Marie probably knows Gabe will be bad, but also knows that once he goes through a 1000 year "rough patch", that he'll save the day again as God's champion.