If the christian holy trinity and the unholy trinity where placed into dnd as it is now, what would happen?

this can be either the old or new testament gods, and this is assuming that nothing caused by either side happened earlier(i.e. genesis), this is also under the assumption that this is the forgotten realms but i'm willing to see perspectives of other worlds(also this isn't for gameplay, this is just the lore)

In FR, the "overgod" Lord Ao (as in alpha and omega, get it?) already fills the role of the all-powerful bearded guy who only shows up from time to time. If he were outright replaced by plain old Yahweh... No clue, or should I say God knows. 🙃 Maybe he would want worship from mortals, unlike Ao who specifically demands that prayers be directed to the rest of the pantheon instead, while allowing the "Unholy Trinity"(some variation of Lucifer, Satan and Beelzebub?) to tempt and torment mortals as a test of faith.

i could definitely see lucifer, satan, and beelzebub as the unholy trinity but the post originally had the dragon(satan), the beast of the sea(the antichrist), and the beast of the earth(the false prophet) in mind

In my worldbuilding there is a god that claims supremacy for good.  His priests lead worship but focus on a facet of creation important in that community.  So an agrarian village would pray for a good harvest, fair weather, protection from blight and disease, and stuff like that.  In another community they might pray for Industrial success, so they might pray for good supply of raw materials, dedication among their apprentices, low losses to banditry and things like that.  But when the priests get together they discuss philosophical theology matters.

In contrast, an underworld god might claim to be the source of easy living at the expense of others, and their priests might pray for their enemies to suffer hardships so they have to rely on the underworld vices to get by.  Their priests might pray for the sorts of torments the local community fears.  And when these priests get together they discuss theology so that it sounds like the other guys until you hear about the details, and then you realize this isn't like the other guys at all.

Technically, in "Dragon Magazine 54" by Ed Greenwood, in the article "Down-to-Earth Divinity", the interloper god Nobanion is named Aslan, from the C.S. Lewis novels "The Chronicles of Narnia". In Polyhedron 115, "Forgotten Deities: Beast Cults: The Lion and the Unicorn", Aslan is said to be a name given to Nobanion in "ancient texts". He apparently arrived in the mid 1000s DR through one of the magical pools in the Weathercote Woods, which could be connected to the Wood Between Worlds from the novels.

Since Aslan is the Jesus allegory, He technically exists in the Forgotten Realms already. I've a character called "Leah the Aslanite" (Human Oath of the Ancients Paladin, 2024) who follows the Nazarite Oath (the same taken by the famous Samson) and wields a club and a sling (Magic Initiate: Druid for Shillelagh and Magic Stone). Uses Find Steed to ride a lion into battle.

Please don't bring in a modern active religion into the D&D world.

This is why I use the Greek/Roman pantheon.

The Pantheon is well written out but the actual religion was never written down. None of the "secret" rites were even written down.

The pantheon can be recolored to cover all the races in D&D.

The religion died out over a thousand years ago.

I agree with TheGnome.  When I was a kid we had these stupid Jack Chick comics where religious nut jobs thought kids were committing suicide when their character died. 

Zero reason to stir up controversy by trying to put religions that people worship in real life into the game.