If you judge the U.S. presidents by the fruits of their actions, we suppose someone could make a case that a number of them were not Christians, regardless of what faith they claimed.
However, Abraham Lincoln was one who specifically rejected Christianity, even writing a pamphlet called Infidelity, which sought to disprove the Bible generally and the idea of Jesus being the son of God specifically.
Later, as a politician, he succumbed to pressure to mention God in speeches but pointedly did not mention Jesus.
Thomas Jefferson attended an Episcopal church but refused to discuss his religious beliefs during his election campaign, and so was accused by rabble-rousing preachers of being ungodly and in league with Satan.
Actually, he believed in the moral teachings of Jesus, but not in his divinity.