Is the issue of a Muslim sitting in gatherings where religion is reviled, and merely denouncing it in his heart, a matter of scholarly disagreement, and what is the most preponderant opinion on it, and what should he do?

One who sits in a gathering where Allah is disbelieved in, while disliking it and condemning it in his heart, is not a disbeliever. There are many proofs for the non-disbelief of one who condemns with his heart, and we are not aware of any considerable disagreement among the scholars regarding his non-disbelief. It is obligatory upon a Muslim to condemn to the best of his ability and not to sit with those who indulge in talks of disbelief and mockery, for Allah Almighty says: "And He has already sent down to you in the Book that when you hear the verses of Allah being denied and ridiculed, then do not sit with them until they enter into another conversation. Indeed, if you did, you would then be like them." (An-Nisa: 140).