Early D&D, inspired as it was (hell, outright birthed) by historical wargames, channelled medieval Europe heavily. Gary's obsession with polearms is legendary; but it was more than that, and what else would we expect from a fantastic medieval wargame? Of course, the fantastic precedes a European mileu by millenia and extends beyond its borders to storied parts beyond. But Europe, with its armored knights and dragons, really stuck...
And part of this was medieveal Christendom, especially in regards to clerics. Now TSR, already aware of a growing panic, wisely avoided statting up Jesus; but its earliest artwork nonetheless depicted many clerics as Christian. From crosses (already effective against vampires) to monkish tonsures, early D&D art had a thing for the Church, doubtless because its faith heavily influenced the medieval expereince as understood by many.
Here's three of my favorites, with thoughts on what they meant to the early hobby:
(1) RESURRECTION. These decidedly Christian clerics are providing a service attributed to the saints. Indeed, healing and/or restoration is emphasized in this tradition...
(2) A DUNGEON DWELLERS MINIATURE. Nothing says "Back in the name of (_______)" quite like a cross shoved in evil's face. There's no doubt who this adventurer serves...
(3) A FUN MEDIEVAL MONK. Played for laughs; but again, there's zero doubt that this is supposed to be the party's cleric. Crosses and tonsures made for shorthand...
Again, this was all about recognition. The fantasy genre by way of gaming was still being created, and historical influences reigned supreme. Magic users still wore robes emblazoned with moons and stars, and fighters wore historically accurate armor and weaponry (Gary's polearms) because this is how these concepts were understood. Fortunately, time and steady mainstreaming changed this; but a certain storybook charm absolutely remains.
And there's more. Much more. Part Water? Sticks to Snakes? Their collective religious influences are obvious. Early D&D was a fantastic medievel wargame, and its understanding of the world was clearly centered thereupon. Even so, Greyhawk had its original deities, including non-human ones from the start; and the game quickly moved past its European roots to encompass the world's fantastical traditions, fertile ground for creative minds...