From my observation on forums in general that have ads, the ads are often generated by the content within the thread. That Jesus ad is on a page in a thread in the comparative religion section. I am guessing that Jesus' name is mentioned in the thread, or something relating to Christianity in general?
The ad on this page right now is for (my) "spiritual awakening." It links to quite a scary site. If you think the Christian ads are bad, you should see the one on "quantum jumping." ;D