A Bible study is not the appropriate place to be proselytizing anyone. It is a place to learn from a teacher and to exchange ideas with your studymates. Ordinarily, such group studies presume that everyone in the group shares a common point of departure about the subject under examination.
Perhaps now you can see why it is generally not a good idea for Catholics to attend Protestant Bible studies. If you are there “holding your ground,” Catholic catechism in hand, you’ll quickly be seen as a disruptive contrarian and may be asked to leave so that everyone else may study in peace. If, on the other hand, you are “there to learn,” then there is danger there as well: Unless you are extremely knowledgeable about your own faith and its understanding of Scripture, what you learn from Protestants about Scripture may quickly dilute your own Catholic faith.
To be fair, it is possible to attend a non-Catholic study, to learn from the teacher and fellow participants, and to respectfully and diplomatically offer your own thoughts on the matter under examination, which might in turn end up being a quiet witness to the Catholic faith. But, at the risk of sounding cynical, I must say that I personally don’t think many Catholics are up to that task – as is demonstrated by the many Catholics who either convert to Protestantism or become “protestantized” in their understanding of Catholicism as a result of such studies. That is why, generally speaking, it is a wiser course of action for Catholics to study Scripture with fellow Catholics under a Catholic teacher.
