A religion isn't a simple thing like a thesis, or a story (at least, not beyond the most superficial level, where, admittedly, people like Angela practice).
So often there is overlap. In many branches of Hinduism, Christ is considered a great teacher and most of his teachings were well-compatible with yoga, vedanta, etc (in fact, the Sermon on the Mount, which vexes Christian theologists is better explained via Vedanta than via Christianity, IMO). Lots of people in that part of the world study and even venerate Christ without identifying as Christians.
Moslems, as you likely know, respect the Old and New Testament. It's part of their faith. There's no reason you can't "roll up" previous faiths like this.
And if you're on the metaphysical end, re: Zen, Vedanta, Trappist, Gnostic, Yoga, Sufi, Kashmiri Shaivite, you eventually begin noticing that everyone's saying basically the same thing. Since metaphysics is less about origin stories and behavioral prescription and more about direct experience, they're all reaching for the same experience, a shift of perspective to a framing of the pure awareness (subject) underpinning the world's apparent diversity (objects).
In the end, it's like language, arising organically and subtly shaded to local custom, but all serving broadly similar function; in the case of religion, to grapple with big questions and to give those who question materiality a way to frame the other (in philosophical terms, for object to come to terms with subject).
As with language, it's not zero sum. You might think French is a beautiful language and still speak Lithuanian. Why wouldn't you?
So far my favorite book on Christianity
The Sermon on the Mount According to Vedenta
Edit: and religion for that matter