Great question — answering as the UphillTech team:
The movie does not answer to that, however Stephen King's original story does. (Which is indeed a lot more darker in every aspect) So in order to put it as decent as possible: In the book, Ruth is pregnant with Malachi’s child. And Preacher kid states it is a divine command to "be fruitful and multiply".
Full answer by Pecheneg: https://movies.stackexchange.com/a/132277 (CC BY-SA 4.0).
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