Odd sci fi book set in far future - "13th" child of a legendary couple

[XayideGoddess]

So in about 2010/2011 I read a book whose main character finds out he is the missing "13th" child of a legendary couple, he finds out he is key in a war between humans who are strictly solipsistic atheists and machines who are highly devoted to God the creator . During the story he is betrayed by a woman who is referred to as a machines "familiar " one of his sister's is on a deep space exploration mission and I think each of the sibling are named after a month which is why the main character existing is such a big deal