The decretal Ex Litteris by Gregory IX (1229) defines the status of the child of a Jewish couple of which the father converted to Christianity: the child must be baptized and raised in the Christian faith. Starting with the commentary of this text by the legal adviser Agostinho Barbosa in the 1640s, I will attempt here to present the norm concerning the children of a converted parent such as it arises from the work of the medieval commentators, underscoring its links to the wider question of the forced conversion of children. During the post-Tridentine period, the decretal Ex Litteris was reinterpreted to allow the converted grandfather, then the grandmother, “to offer the child for baptism,” even in the Jewish father’s and mother’s lifetime. This normative work is related, in particular, to pressures that the Church exerted to convert the Jews in the Papal States. The evolution of the doctrine is punctuated by a series of cases: the case treated by Martin de Azpilcueta under Gregory XIII, then the case settled by Benedict XIV in 1751. The papal clarifications show, over the long term, a hardening of norms to the detriment of the Jews.