The Torah declares: "And every uncircumcised one shall not eat of it." The Mekhilta asks a pointed question: why is this verse necessary at all? The Torah already stated "No stranger may eat of it," which seems to cover anyone outside the covenant. What does the additional prohibition add?
The answer targets a specific person the first verse might have missed: an uncircumcised Israelite. A Jew who, for whatever reason, had not been circumcised would not qualify as a "stranger." He was born into the people of Israel. He was no foreigner. The general prohibition against strangers eating the Pesach (Passover) would not obviously apply to him.
Without this second verse, someone might reasonably argue that an uncircumcised Israelite — being fully Jewish by birth — retained his right to participate in the Passover sacrifice. The verse "every uncircumcised one shall not eat of it" eliminates that argument. The word "every" is absolute. It does not matter whether the uncircumcised person is a foreigner or a native-born Jew. Without circumcision, the Pesach is inaccessible.
The rabbis understood this as a statement about the indivisibility of covenant identity. Being born Jewish is necessary but not sufficient for full participation in Israel's most foundational ritual. The physical sign of the covenant — brit milah, the circumcision commanded to Abraham — must be present on the body. Lineage opens the door to the covenant, but circumcision is the threshold you must actually cross.