A note before beginning: Even the term "religion," is contested, but before proceeding I want to disregard the etymology for the term being from "ligare". Make no mistake, this is entirely acceptable, but is used often in contemptuous polemics without any real truth to it. A friend on here (no citation) more accurately suggests that it's from proto-Italic "care" instead of that dubious reconstruction.
And though this stands, I should also indicate that the term "religion" is being used here as a sort of shorthand. There is some accurate sense, understood best in Simone Weil, that indicates what religion shouldn't be—it's not the same way I use it here. Think of it as not unrelated to, but the raiment of faith.
Incipit. I think I've seen every side of ecumenism: charitably for an invisible church, as a heresy condemned by Orthodox hieromonks, & lone perennialist projects suggesting a "deep" version of it. However, I think honestly that there's something left unexpressed, that's obvious to anybody in conversation: There's absolutely nothing in common between one religion and another.
There are scholars of comparative religions; very influential ones at that, which I am sure you have heard of. Furthermore, the way I was taught religion by my public school was in spheres of "world religion," having more in common than otherwise; and indeed in ninth grade, our only school field trip was to a mosque, synagogue, and Buddhist & Hindoo temples. I seem to recall an episode where somebody told me the three "Abrahamic" religions were founded by three sons of the Hebrew Bible; I checked, and the only way this could square is if Abraham founded Judaism, Jacob by relation to Christ, and somewhere is Ahmad related distantly.
But throughout all this, watching risible Vatican-flag types laud Sharia law, there seems to be a distance that I can't express. It's my conception that there is absolutely nothing in common between a Christian and a Muslim; and the distance is only expressed further by positing similarities.
Badly timed update: It's in hadith that Ahmad is related to Abraham.