6 The next verse too is suspicious, "If you believed Moses, you would also believe me;" for the religion of Moses is so entirely different from that of Christ, that if the Jews believed one, they could not believe the other. Moses strictly forbids any work to be done on Sabbath, and gives as a reason for this prohibition that God made the world and all that is therein in six days, and rested on the seventh day, which is Sabbath; and therefore blessed or sanctified it as His haven of repose after toil, and commanded that breaking the Sabbath should be punished with death.
The Jews, in obedience to Moses, insisted strongly on this, and so would not even listen to Christ when He told them that God always works, and that no day is appointed for the intermission of His pure and unwearied energy, and that accordingly He Himself had to work incessantly even on Sabbath. "My Father," he says, "works always, and I too must work." Again, Moses places circumcision among the rites pleasing to God, and commands every male to be circumcised in the foreskin of his flesh, and declares that this is a necessary sign of the covenant which God made with Abraham, and that every male not circumcised would be cut off from his tribe, and from his part in the inheritance promised to Abraham and to his seed. In this observance, too, the Jews were very zealous, and consequently could not believe in Christ, who made light of these things, and declared that a man when circumcised became twofold a child of hell. Again, Moses is very particular about the distinction in animal foods, and discourses like an epicure on the merits of fish, and birds, and quadrupeds, and orders some to be eaten as clean, and others which are unclean not to be touched. Among the unclean he reckons the swine and the hare, and fish without scales, and quadrupeds that neither divide the hoof nor chew the cud. In this also the Jews carefully obeyed Moses, and so could not believe in Christ, who taught that all food is alike, and though he allowed no animal food to his own disciples, gave full liberty to the laity to eat whatever they pleased, and taught that men are polluted not by what goes into the mouth, but by the evil things which come out of it. In these and many other things the doctrine of Jesus, as everybody knows, contradicts that of Moses.
Source: Reply to Faustus the Manichaean (New Advent)