Weddingbee boards

My husband and I have this casual friend, "Bill."  We hang out with him or with his fiance and him maybe once a month.  We also play Wii Mario Kart together online maybe once a month.  We met Bill through work; he's one of the only people at our company who is our age, so we have a lot of things in common... on the surface.  We get along pretty well, but the friendship is very casual.  We mostly stick to lighter topics and common interest activities, and we have never discussed religion, politics, etc... save a stray comment now and then.

The problem is that since I've know Bill (since last October) he has been continuously asking us to attend his Bible study group.  I converted to Catholicism from a church much like the church he attends about 1.5 years ago.  It was not a decision made lightly.  I know (mostly because of the stray comments he has made) that we have VERY different views on major issues.  I am not really interested in attending his Bible study because 1) I used to attend a similar program and no longer believe their core beliefs, 2) I feel that this group will probably not receive my views well, 3) I would be attending out of a courtesy to my friend, not because I am interested in listening to their message.  To make things further complicated, I would probably go if he belonged to a different church or religion.  I generally think religion is an interesting topic, and I took several theology classes in college.  I also have a sneaking suspicion that Bill is trying to convert us, and he doesn't really care about discovering my personal beliefs.  I would be interested in discussing our differing views, but not in a situation where I would be basically defending myself in front of a group of people I don't know.  Also, my husband is pretty laidback and doesn't care whether or not we attend the Bible study. 

I'm not sure how to tell Bill that I want to keep our friendship casual.  I've basically just been making up excuses to this point, but I think it's starting to hurt his feelings.  I don't want to ruin our friendship, but if we were dating I would tell him I wasn't ready to commit to a monogamous relationship.  How do I translate that message into a friendship?