" It's difficult to ascertain and judge any Christian religion if it isn't against or in contrast to the earliest one."
From a Christian perspective that seems irrational. A religion claiming to be Christian should best be contrasted with scripture, not other denominations. A long history does not certify "truth" or holiness or rationality. Though I hold slightly different beliefs, I associate with a General Conference (Atlanta) fellowship. No one within that fellowship cares about Catholic opinion. They care about Scripture.
From a historian's perspective contrasts and conflicts are important. So while I might record shifting Catholic viewpoints of Protestant faiths, Catholic beliefs do not certify or call into question other faiths.