Atheism in the DnD5e universe: is it a thing, and how common is it? Are clerics accepted to derive power from the divine?

My question comes up from a bit of weird exchange we had last session. Short version of the question: Is not believing in the existence of gods in most DnD universes really a widespread thing? Are clerics that can tap into these abilities very uncommon or believed to get their magic from some other source? And if no to these, how does one handle PC’s behaving as if this were so?

Long version: I ended up multiclassing cleric through a pretty cool set of story hooks that my DM did a good job working with me in setting up. I definitely wasn’t a stereotypical stoic priest preaching about my god or anything like that.

In terms of story as it played out when I revealed this new power to the rest of the party (which they were present OOC for me receiving), one of our party had a roleplay situation that came where I could use my new class to help, and I told the rest of party that if they would help me, I’d like to offer this help to the other PC (the object of the help was not present IC so didn’t react).

I was kinda bummed that the reception was anywhere from condescension to incredulity (both IC and OOC), at least one of them heavily implying that thinking a god exists is stupid, and I really felt like this was out of place; my impression (granted, as someone fairly new to this) was that the existence of gods in the DnD universes are no more contested than the existence of magic: it exists, everyone knows it exists whether or not they’ve directly been affected by it. Is this impression wrong? I feel as though nobody would have blinked if I multiclassed into warlock and made a pact with a supernatural being, but for some reason because it was explicitly a ‘god’ I attuned to there was a lot more incredulity.

I’m not really sure how to handle this going forward. Is this the sort of thing I should talk the DM about and get straight? Should I talk with the other players directly? Is my impression on the wide acceptance of gods just wrong? Not sure how to deal with this, IC or OOC. It sorta sucks, because I was really excited for this multiclass and I walked away feeling kinda dumb and not really being into it anymore. Thanks for suggestions.

E: Should have mentioned, we are using homebrew that seems to get material like this from Forgotten Realms based on our warlock’s already-existing pact. DM hasn’t mentioned to either of us to tailor our choices of being to his setting.