4 “I have done one work, and you all marvel.” And immediately He subjoined: “Moses therefore gave unto you circumcision.” It was well done that you received circumcision from Moses. “Not that it is of Moses, but of the fathers;” since it was Abraham that first received circumcision from the Lord. “And ye circumcise on the Sabbath day.” Moses has convicted you: you have received in the law to circumcise on the eighth day; you have received in the law to cease from labor on the seventh day; if the eighth day from the child's birth fall on the seventh day of the week, what will you do?
Will ye abstain from work to keep the Sabbath, or will you circumcise to fulfill the sacrament of the eighth day? But I know, says He, what ye do. “You circumcise a man.” Why? Because circumcision relates to what is a kind of seal of salvation, and men ought not to abstain from the work of salvation on the Sabbath day. Therefore be ye not “angry with me, because I have made a man every whit whole on the Sabbath day.” “If,” says He, “a man on the Sabbath day receives circumcision that the law should not be broken” (for it was something saving that was ordained by Moses in that ordinance of circumcision), why are you angry at me for working a healing on the Sabbath day?
Source: Tractates on the Gospel of John (New Advent)