I have finished my Phd in maths shortly. A few days ago, I have heard a revision decision from a very good journal where I submitted my paper. The comments from referees are generally fairly positive, and are not too difficult to address. However, I have found a mistake in a theorem proof, which the referees have not spotted. This mistake is difficult to fix, or may be unfixable. The theorem is the extension of the main result of the paper. I assume it is not an important result, as there are no comments or revisions from referees on this part. Then how to write to the referees to address this issue? I imagine it would annoy both the referees and the editor. When I am going to submit the revision, is it better to remove the wrong theorem or to keep it there? Many thanks.