Problems of Strategy: "To put the two statements together we may cite the Chinese adage, “Nothing in the world is difficult for one who sets his mind to it.”

So it isn’t a Mao quote he is quoting an old adage.

If you want to quote Mao the second half is the good part: "To cross the threshold is not difficult and mastery, too, is possible provided one sets one’s mind to the task and is good at learning."

A good materialist always hedges statements with "if"s and "possible"s