I love, love, loved MOPS. It has been one of the biggest, single blessings in my life. One of my best friends to this day is from my first table of women.
Our table had mainly Christian women, but also Jewish, women in interfaith relationships to Muslim, agnostic, etc. To answer your question, you do not have to be any certain religion, although the majority are Christian.
The group I joined was large and focused on having an educational talk followed by a craft. My sister's group had an educational or motivational speaker followed by an exercise class. I think the bigger the better. We moved and I tried joining a smaller group, but it ended up being a one woman show....she was super controlling about everything, funny as heck, but it was all about her, which got boring after 3 mos.
It really helped me be a better mom and wife in sooooo many ways. Go for it!!