Let's say you made up a religion based on Pokemon — such an undertaking would probably require you to look at the lore of the game. As far as the Pokemon world is concerned, Arceus is the Pokemon that (possibly) created the universe. A legendary god, if you will.
The "Mewsus" Twitter account takes that lore and runs with it — the result being pretty funny for those of you who know a thing or two about the Pokemon games. Just pretend Mew is like a Pokemon Jesus and go from there: