Because when Catholicism had almost complete control of Christianity, they controlled the theological narrative and all protestant thought comes from Catholic thought. The first protestants originally grew up in an entirely Catholic world and other early forms of Christianity that did have different views had been obliviated. So all there was was Catholic thought which was just implicit within life and was something you would never even think to question. For example, trinitarianism is not mentioned as a doctrine anywhere in the Bible but it was adopted by Catholicism and subsequently by Protestants, and that's just one of probably many examples. So if practically all early forms of Christianity other than Catholicism were lost making Catholicism the only major survivor, and then protestantism was a tangent from Catholic thought, is it fair to say that, although Protestants like to interpret the book in their own ways, they are ultimately descended from Catholicism since they inherit much of the theology, other non Catholic forms of theology being lost to time?