They were Christian, but in contrast to European Christianity, the Founders were highly Philo-Semetic (Jewish Loving).
George Washington visited the Touro Synagouge in Rhode Island and sent letters to all American Hebrew Congregations as a sign of harmony. Alexander Hamilton attended Jewish schools. Ben Franklin contributed to Congregation Mikveh Israel in Philadelphia. And as for John Adams, he wrote:
"[Jews] are the most glorious nation that ever inhabited this Earth. The Romans and their Empire were but a Bauble in comparison of the Jews. They have given religion to three quarters of the Globe and have influenced the affairs of Mankind more, and more happily, than any other Nation ancient or modern."
Even American institutions such as Thanksgiving (Sukkoth) were adapted from Jewish rituals and festivals.
It's funny, when I was growing up, "Judeo-Christian" was the politically correct way to refer to our heritage. Now it's taboo among the Muslim-loving left, and passionately supported by the virtuously Philo-Semetic Right.