I've spent a few months on medical missions but all of mine were with Christian organizations. I did go with Medical Ministry International, www.mmint.org. They allow non-Christians to volunteer and it certainly wasn't uncommon to have non-Christians or people of other religions work with us. I think they are an excellent organization and you can be valuable even if you're not involved with the "ministry" part. We appreciate all hands and everybody is very friendly. I've been with the Dominican republic team a few times and it's wonderful. The Latin American choices are good simply because airfare is cheaper. I would really encourage you to check it out, even though it is a religious organization.