lol, OMG, I got the answer. I have such a terrible memory, it's not even a manga. Though, I do read dozens of these stories in a month.
"7 Transmigration Tips From the Saint" it's a Web Novel on Trans-weaver
I have such a vivid imagination that the cover art alone created images in my head from reading it, lol.
It might not be your thing. I read it originally as it was being translated, now I just want to re-read it all at once and see if it holds up.

I'm so DESPERATE, seriously.
I need help finding this and this story is similar so maybe you lovelies know it. It feels like it's been removed or something.
It was a BL [90% sure] with the MC being transported to a fantasy medieval world by a god and he was immediately praised by the temple as their Only Saint, chosen by their God. He was spoiled and treated well.
But the MC doesn't quite like this treatment, he likes to have some work so the god gives him silly little tasks with a Game/System window but also just visits him directly in person.
It turns out that the God was actually the MC's lover in a past life, a thousand years ago in the fantasy world or something but the MC was killed because of a plot/war while the ML was made into a God.
The MC occasionally has flashes/dreams of this past life and the ML is really invested in making MC remember their past life.
There's a section where the ML takes human form for an extended time and travels with the MC to some temple outside of the capitol for some ceremony or something.
Another part where the MC pretends to be a novice priest and goes to a training island with other novices but the boat they're on is attacked by a rival religion's zealots.
Also when the MC finds out that they had a history he gets really upset with the ML and says that he doesn't want to be with him if the ML only wants him because of their past life. he wants the ML to accept him as himself, not as this past version of him.
**Also, the first time they did the DEED, there was like an Aurora and a bunch of miracles, ya know because the ML was so happy his divine power covered the city.
I think the cover had the god/ML in a kind of revealing robe and the MC in a standard brown Saint/Priest robe.
This is SO ANNOYING. I've been combing through the list without filtering anything out [super annoying with all the DJs clogging it up] and I STILL can't find any cover that's vaguely similar to what I remember.
Heck, maybe I'm mis-remebering what the cover looked like.
HELP! ╥﹏╥ ╥﹏╥ ╥﹏╥