The use of "heretic" is a particular term. Maritain was never declared a heretic. But his late in life final writing about "Hell being empty" -- to sum in it up in a nut shell -- well, those ideas were assuredly not orthodox and were remarkably irresponsible for a Catholic theologian who has no right or privilege to invent doctrine contrary to the Magisterium.
On another sour note, Maritain and Saul Alinsky were commrades.