It depends on how you define the words. Is omnipotence the capacity to do absolutely everything? Then yes, God is not omnipotent. But no philosopher or theologian ever defined omnipotence like that. Using that against Christianity is a strawman. Omnipotence, clearly defined is the capacity to do anything possible. So God can't make a squared circle but he's still omnipotent. He can't create a universe where everything follows his will and still is free at the same time, but he's still omnipotent.