The whole "Doom is a Christian game" thing was funny for a long time but I think it's funny that Doom Eternal and its DLC kind of seriously squashed that whole view of the series. Even if the whole thing just started as a meme in response to a few people back then saying Doom was offensive for having demons.
"Doom is basically pro-Christianity, since you're killing demons-"
(Angels are actually just glorified narcissistic robots, they are not very nice and we should kill them, Heaven actually isn't any better than Hell)
"Doomguy is basically a Crusader though, don't you get it-"
(The angels are lying assholes and the afterlife isn't real, they just want to exploit your blind faith to feed their egos and keep themselves alive)
"Those aren't the real angels though, Doomguy is Catholic-"
(Doomguy literally shatters the soul of God)
"You just don't get it. Doom is actually based and Christian-"
(TAG2 reveals God is actually also just a robot who betrayed Satan, who was the REAL God, then tricked everyone into thinking he was God, so actually everything bad is kind of his fault)
If anything the modern Doom story has very staunch anti-religious themes. The Maykrs are the source of basically all Abrahamic religion in Doom's canon and they are absolutely terrible, with all of those beliefs entrenched in deliberate lies to exploit humans. The Slayer isn't on the side of the angels because they're "good," they are not good, and he isn't gonna help the Father just because other angels think he's good.
Doom posits the only thing worth fighting for is humanity. Hence the whole "The Ancient Gods" title. The Ancient Gods are absolute bullshit and we should not pay them any mind. They suck so bad and all they've done is ruined lives and made everything worse. Humans have to carve their own path and fight for themselves. They even spell this out with Thira's speech in The Dark Ages:
“We will always be a people of faith. But we must consider now who we will put that faith in. And I say to you there is no better place to turn to... than ourselves."
In Doom, religion is just another expression of authoritarianism. You have to have faith in yourself, because the gods who beg for your faith will only use it to fuel their egos.