Acceptance of the Mitzvot as a Requirement for Conversion

 https://www.etzion.org.il/en/acceptance-mitzvot-requirement-conversion

In any event, other Rishonim have explicitly ruled that acceptance of the mitzvot is indeed an indispensable requirement for conversion. Thus writes the Shulchan Arukh:

 

All matters pertaining to a proselyte - informing him of the mitzvot that he may accept them, circumcision, as well as immersion – must be [performed] in the presence of three who are fit to judge and during the day. This, however, is only lekhatchila, but bedi'eved, if [the proselyte] underwent circumcision or immersion in the presence of two or at night… he is a [valid] proselyte and may marry a Jewess. This is with the exception of accepting the mitzvot, which invalidates the conversion if not performed during the day and in the presence of three [judges]. (Shulkhan Arukh 268:3)[6]

 

            In actual practice, more recent authorities have been inclined to rule that acceptance of the mitzvot is an indispensable requirement for conversion. Rav Goren relied on this ruling even when it led to a leniency. He ruled that a certain set of siblings were not to be considered mamzerim, because their father, who claimed to be a convert, had never properly accepted the mitzvot, and so he was not a Jew.