I think you've received some pretty good advice here.
My beliefs in this area are that 1. two committed people attempt to support and strengthen each other's extended family relationships. 2. The couple discusses their feelings together, and act as one when there are issues involving extended family. 3. They protect their union and one another. No one takes any precedence over the two. 4. The primary decision-maker and mouthpiece in disputes with extended family should be the blood relative...although decisions must be agreed upon with the marital / relationship partner.
1. Present the problem to your husband, but take care that you don't steamroll him. From your post, it seems you care about his feelings, you don't want to push the mother completely out of his life, and you and he seem to agree on the problem.
2. Practice compassion for her. Put yourself in her place. This isn't so she can "get her way," but to help you not be cruel. One day, your children will marry. You will be in this position. Think about how you will feel when you lose your role with them, and how you hope they'll treat you. Remove your feelings about the religion issue for now. That's separate. This will help you understand and appreciate when she cooks for your anniversary and such things. You'll understand why she is a bit desperate to be a part of some of your celebrations. You may be more compassionate, which may help your internal feelings, your husband's and MIL's.
3. Decide with your husband how you'll approach issues that bother you.
a. If MIL criticizes you to him, he should immediately and firmly tell her that he is not going to listen to criticism of his wife. "If you have a problem with ____, you need to discuss it with her. If you two can't work it out, then the three of us can sit down, but you are not welcome to complain to me about her when she's not present." If she continues complaining, he should interrupt by saying something like, "Think seriously about what I just said. Hope to talk to you again soon. I love you. Goodbye." Hang up.
b. When she begins to bring up religion, the very next sentence should be from her son, "Mom, _____ and I have tried to be patient when you make comments like that, but we need to tell you that lately because of the way you speak to us about your religion that it is becoming more difficult to be around you. Listen carefully. _______ and I will never convert. What you have to do now is either accept that so we can enjoy more time with you - or know that we won't be spending very much time together and will cut short visits when you talk to us about converting. Starting now."
If she continues, give her another warning, "Mom, our visit will have to end right now if you can't stop trying to convert us. (Offer some change of subject - move to the den, start talking about your work, one of her friends, the grandchildren's interests at school...)
If she continues, end the visit. If she's at your house, you can stand up and say, "Let me help you get your things together." or "I'll see you out. I hope you'll think carefully about what I said."
See her out.
Train her on what you'll accept. Just make sure your husband is emotionally on board.
She has a place in his life, but not in your marriage. She may not know how to articulate it. It is HARD for a loving mother to re-position herself in a son's life after he gets married. It is one of the most loving gifts from a wife to make a sincere effort to protect and support a MIL /FIL / children from previous marriage relationship. Sad when it can't be done. Good luck!