Watts was a Christian.
In fact, he was an ordained Protestant minister. Although he never incorporated this into his popular philosophy.
Neither is it inconsistent that he was the leading American popularizer of Zen Buddhism. Buddha himself never claimed to be divine, and there is no reason (in Classical Buddhism) why a person cannot be a Buddhist and follow another, or no other, faith. Buddhism is a philosophy of life, not an explanation of the universe.
I like Sartre too, so I guess you could call me a Zen Existentialist.