First of all, I am gay. My first boyfriend ws Catholic, although he never went to church, never paid any adherence to religon at all. When I made soem remark about the Pope he went ballistic. The relationship didn't last after that.
It is best toi find otu about their religious views before you get in too deep.
I was in a relationship (married) when I became an atheist. We were both Christians when we got married. At the time, I was questioning Christianity --- not so much the existence of god. I shared my findings with him during my deconversion period.
I didn't think it would be an issue because he really wasn't that devout. But, when I eventually became an atheist, that was the last straw --- he said we were "unequally yoked." Our marriage ended shortly thereafter. The most awkward part was that he pushed me away.
it's mostly okay. he believes there is a god. he has no clue what kind of god; he hasn't read the (jewish) bible, and he is shocked whenever i read him bits because it's so ridiculous. when i want to cut him short i just say "talking snake." he honors his parents on their yahrzeit. let him. he's not hurting anyone. he doesn't like it when i say i am an atheist; well, he knows i am, so i don't have to announce it every minute, but sometimes i tell him about something i found here on this site, and i think he is kind of processing that. he has alzheimer's. i pick my battles.