The Torah states a blunt exclusion about the Paschal lamb: "No stranger may eat of it." The Mekhilta explains who "stranger" includes, and the answer is broader than it first appears. Both a heretical Jew and a non-Jew are subsumed under this prohibition — two very different categories of people, united by a single disqualification.
The proof text is (Ezekiel 44:9): "And every stranger, uncircumcised of flesh and uncircumcised of heart, shall not enter My sanctuary." Ezekiel's prophecy about the future Temple draws the same double line: those who are physically uncircumcised (gentiles who have not entered the covenant) and those who are uncircumcised of heart (Jews who have abandoned the faith) are equally barred.
The Mekhilta's pairing is striking. A gentile is excluded from the Passover because they have not entered the covenant of Abraham. A heretical Jew is excluded because they have betrayed it. The physical act of circumcision is necessary but not sufficient — the heart must be circumcised too. A Jew who has turned against the Torah is, in the Mekhilta's framework, as much a "stranger" to the Passover as someone who was never part of the covenant at all.
This teaching defines the Passover sacrifice as the ultimate loyalty marker. It is not simply a ritual meal — it is a declaration of covenantal belonging. To eat the Paschal lamb is to affirm that you are part of Israel, bound by Israel's covenant, committed to Israel's God. Those who reject that commitment, whether by birth or by choice, are excluded from the table.