have you tried to join a Christian web ring? that might bring in more members for you. I joined one with my forum and just got approved today. it's just a thought.
here is the one I joined http://l.webring.com/hub?ring=christianwebring
I joined the Christian Webring and asked them to feature the forum. Hopefully they'll do it. I also submitted the site to various search engines and to the DMOZ directory(it will get a good ranking if approved ). Pls keep inviting your friends too.