They tried to hide it... Oh, did they ever. Marx was obvious; but he was from Germany, so what if he was a Jew! Lenin, he was all theirs; Lenin was Russian (oh, sure, a mixture from different regions, a little from the North, from the East, some from the South, from the West). But a Jew? Never! Never!
Yet another fable created by the Soviet party machine.
Here's what we know: Lenin's great-grandfather was a Jew. Therefore, his maternal grandfather, Israel Blank, was a Jew — a Jew who got baptized in his teens and was renamed Alexander. So there was Jewish blood in Lenin, at least a quarter; and that quarter was large enough for his successors to adamantly try to conceal it.
However, if you look at the list of Old Bolsheviks pseudonyms aside, it reads like the guest list for a bar mitzvah: Rosenfeld, Bronstein, Kaganovich. So Lenin was actually a minority! There is even evidence that he was definitely more proud of his Jewish roots than future generations of communists would ever let on.
So much for trying to hide it.