Let’s try another approach. You and your spouse have differing ideas, different likes and dislikes, different hobbies, work, cars, what have you. Yet you are one. You both have different needs, different tastes in food, clothes, but you are still one. You are united, visibly and invisibly.
Indeed we are. But you wouldn't say that if the couple in question was divorced. So, what is the difference between being divided into many different denominations, and schism? How do you know that the different denominations are not "divorced" (so to speak), i.e. in schism? What you call unity is indistinguishable from schism.
-A8