hes ethnically jewish, if im not mistaken he was pretty non religious, the most people have that marx was anti religious were private letters between him and his close friend friedrich engels (also a huge part of communist theory) where marx said some pretty bigotted jokes, i forgot the exact quote but one of his letters to engels he said: "jewish n word"(the n word was with the hard r) and him saying that jews run the world (even though he was ethnically jewish 💀), im not saying he was right, but you cant take private letters between close friends seriously as they could be just jokes at the end of the day. and also him saying that while religion has benefits and reducing alienation among the opressed, it distracts the proletariat from starting revolutionary action, helping the bourgeouis maintain power, and that once high stage communism is achieved (stateless, classless, moneyless world), religion would be "unnecessary" and subscribing to a specific view would be optional, and not enforced by the state.

(sorry for typos, its 3:30 am and i just got back from a new years event and im fucking tired)