Being Religious Without Belief in God
Question
I recently had a halacha question that i asked a friend much more learned than me and he gave a wonderful answer; full of details and citations. We continued discussing it when all of a sudden he tells me he does not believe in God. I was shocked for a moment because for the last 20 minutes we were discussing different rabbinic opinions and not only that, he keeps shabbat, kosher, wears a kippah and was even growing his beard out for the omer. But he doesn't believe in God. Blew my mind. It doesn't seem like its from a lack of emunah per se but just that there is no solid proof of God's existence.
I studied philosophy, specifically ethics, in college at the same time as I started learning Torah. When it comes to arguing for God's existence, I have all the arguments ready to go from Creator -> God ->HaShem and im very passionate about it. The beauty of mitzvot? I can talk about those too. I've continued my Torah and philosophy studies for a few years after college, even now, as well but I never ran into this situation where someone keeps the mitzvot so well, knows quite a lot, but does not believe in God. I want to keep expanding my knowledge and ability to converse with different type of people but I have no idea what field of Torah/philosophy this would be. I thought about perhaps learning more about emunah but emunah has an emotional root to it and I believe my friends case is more of an intellectual one. Is there something else besides emunah that I can read up on?
Does it make sense to ask: Outside of the 13 principles of faith and first commandment, can a person be religious/observant if they don't believe in God?