Well, they took inspiration from Clement Clarke Moore's (1922) "Twas the night before Christmas" and illustrations from Thomas Nast (1881) showing him in red with the sack over his shoulder. Certainly, Coca Cola has solidified the modern image with their ad campaign though starting in 1931. Src
I think you don't get it, modern Santa is so far away from his origins, at the point is not a christian figure anymore. I mean, christmas elves? A flying pack of reindeer? To me, is just a very happy and oversized leprechaun. Not very christian if you mind me lol. Not bad per se, but it's good to keep in mind christian symbols are different than this red hat.