I have a PhD in STEM. I started believing in my late 20s in graduate school. Readings that were influential included: accounts of high profile scientists who were also believers (e.g., Freeman Dyson), Tolstoy's "Confession", poets like William Blake and thinkers like Simone Weil, philosophers like Schopenhauer and Kierkegaard, and apologetics from C.S. Lewis and GK Chesterton.
My motivation to read any of this was downstream of some other internal feeling that I couldn't shake and slowly began to gnaw at me in my 20s -- that what I could sense around me (or sense at all) couldn't be all that "is". I suppose one way to phrase this is that I became increasingly disturbed by my inability to answer fundamental "where?" or "why?" questions (e.g., "why did the Big Bang happen?", "where is the singularity?", etc.). The standard retorts that some things are simply mysteries didn't satisfy me. Instead I started to suspect that much of what I thought was "territory" was actually just various "maps" that people have created in their minds to help navigate the territory. Around this time I stumbled onto Immanuel Kant's antinomies and realized that many people had thought along these lines in the past. Once I was on this trail, I've never strayed.