"By diffusion" like how Norse people had crosses and mjolnr at the same time without seeming contradiction, or how folk religions incorporated Jesus without being exclusively "Christian"
Someone was asking whether veneration of the saints have anything to do with Roman gods. So to prevent follow up inquiries concerning this, I would like direct them to this useful answer by /u/thejukeboxhero http://www.reddit.com/r/AskHistorians/comments/2xlol3/did_the_veneration_of_catholic_saints_directly/