What does Thomas Jefferson mean when he said "Christianity neither is, nor ever was a part of the common law"?

"Christianity neither is, nor ever was a part of the common law. In every country and in every age, the priest has been hostile to liberty. He is always in alliance with the despot, abetting his abuses in return for protection to his own. Among the sayings and discourses imputed to him [Jesus] by his biographers, I find many passages of fine imagination, correct morality, and of the most lovely benevolence; and others again of so much ignorance, so much absurdity, so much untruth, charlatanism, and imposture, as to pronounce it impossible that such contradictions should have proceeded from the same being. And the day will come when the mystical generation of Jesus, by the supreme being as his father in the womb of a virgin will be classed with the fable of the generation of Minerve in the brain of Jupiter."