Who was the first Jew to play in the NFL?
Arnold Horween, but there is a caveat. We'll get to that.
The first NFL games were played on October 1, 1922, with such magical battles as the Dayton Triangles and the Oorang Indians, the Toledo Maroons and the Evansville Crimson Giants. There are some team names you would recognize, including the Chicago Cardinals (now in Arizona, natch).
The Cardinals quarterback was one A. McMahon. His real name was Arnold Horween, who previously starred at Harvard. Horween led the Crimson to the mythical national championship, as well as the only bowl game in its history. Now in the pros, he decided to hide his name (mind that it was already disguised from "Horwitz") to protect his family from the horridness of their prodigy playing professional sports.
In the club's second game, Horween scored two touchdowns, leading Chicago to a 16:3 win over the Green Bay Packers (another name you would recognize). Overall, he played three years in Chicago, and was the Cardinals player/coach during the final two.
Alright, let's get to the caveat. Arnold's brother, Ralph Horween (aka R. McMahon), also played for the Cardinals. But he didn't play in every game in 1922, so we can't be sure that he participated in that first contest.
Moreover, considering the Horweens hid their identities, could there be other secret Jews among the original NFLs?
So let's revise our answer:
Arnold Horween... and, possibly, some others.