It's important to note that religion played no part for any of the characters in the '60s. And Nightcrawler wasn't even invented until the mid-70s and Giant Size X-Men #1.
In the case of The Thing, they could never outright portray him as Jewish until the 2000s.
People guessed that he was Jewish because his way of speaking and background from the Lower East Side and being a guy with working-class origins and affect, while the somewhat Ashkenazi name of Ben Grimm (the surname being Austrian or German in origin) implied that he was Jewish. And Jack Kirby himself confirmed that he saw Ben Grimm as Jewish when he sent a fan a postcard with this drawing (by him) in the mid-70s, wishing them a Happy Hannukah.
People have pointed that Ben Grimm looks like the famous Golem, a figure in Jewish folklore. And of course, Ben Grimm shares a name with Jack Kirby's own father, Benjamin Kurtzberg who fled Austria to the new world to escape anti-semitism.
But ultimately, the first openly Jewish superhero was Kitty Pryde, her writer, Chris Claremont is Jewish himself and she wore a Star of David around her necklace.
https://www.cbr.com/first-jewish-superhero/