If Catholics would stop trying to pawn their religion off as the only true ‘church’ of Christianity there would be no more to oppose than we’ve done in the past with Mormonism. But the insistence that only in the Catholic Church is there salvation requires Christians to oppose such heresies as are at the heart of catholiciism.
The singularity of Catholic Dogma is one of it’s strongest truths. Jesus has one flock. You are with Him or against Him. The numerous Protestant groups can’t all be right, because there is only one truth. It might be nice to imagine that we can all believe different things and all be right, but such a notion defies logic on its face.