Which ones have you found so far?
I would start by googling religion forum or christianity forum or terms like that.
I would also get out of the mindset of going to a forum with the intent of promoting your products.
You should join forums that you feel you want to be a part of. If you go to a forum (or a blog) just to post your link, it's likely to be seen as spam, even if they allow "posting".
However, if you join a community and actually contribute, participate, comment, and interact, then you'll find that if the online community allows commercial links in the signature, fellow members of the community will click on your link if it's relevant to their interests.
You may even be able to sponsor contests or giveaways within the community in order to help get your name and (and help give back to the community).
It can take a LOT of research and reading to promote in blogs and forums. It's not just signup, post your link, move to the next site
Need help finding Religious T-shirt blogs or forums 
