Rabbi Nathan found a specific legal scenario embedded in the verse "let all of his males be circumcised." The phrase excludes a particular case from preventing a master's participation in the Pesach (Passover) offering: a servant who immersed in the mikveh (a ritual immersion pool) for the purpose of conversion before his master did.
In Jewish law, when a non-Jewish servant underwent ritual immersion for proselytization, the act carried significant legal consequences. If the servant immersed before his master — that is, before the master had formally initiated the servant's conversion process under his authority — the immersion effectively freed the servant. An act of religious transformation performed independently, without the master's direction, severed the bond of servitude.
Rabbi Nathan ruled that this now-free former servant does not count against the master when determining eligibility for the Passover sacrifice. The verse about circumcising "all his males" refers to males currently under his authority. A servant who gained freedom through premature immersion is no longer "his male" at all.
This teaching reveals the transformative power the rabbis attributed to conversion rituals. A single act of immersion could rewrite a person's entire legal status — turning a servant into a free person, dissolving a master's property rights, and reshaping who counts as part of whose household. The mikveh was not merely symbolic. In rabbinic law, the waters of immersion had the power to break chains that no human court had loosened.