The Mortara family was Jewish, and lived in Bologna, then a city of the Papal States.
Catholic teaching holds that baptism completely transforms a person. He is “born anew” or “born again” Jn 3:3. Because this sacrament was given by Rabbi Yeshua himself, even Pope Pius IX said that his hands were tied, he could not alter it.
Baptism also opens the door to eternal life. Once a person is baptized he is irrevocably part of Rabbi Yeshua‘s family, just as circumcision welcomed Abraham and his descendants into God‘s family. Gen 17:13–14.
Papal States Law
Papal States law held that, to protect a child’s immortal soul, if death appeared imminent a Catholic nanny caring for a small Jewish child was required to baptize the child. Once baptized, Papal States law held that the child was irrevocably a Catholic.
So Papal States law prohibited a Jewish family from hiring a Catholic household servant or nanny to prevent this situation from arising. If there were no Catholic living in the house there would be no baptism and the Jewish child would remain with his parents under Jewish law.
Observant Jewish families, however, found it convenient to hire Catholic household servants because they were not subject to Jewish law. The Jewish families could hire pagan household servants, but in the Papal States Catholics were far more numerous than pagans so many Jewish families simply hired Catholic nannies anyway, the moral equivalent of driving without a safety belt.
Jewish families blamed the Catholics for adhering to their own law when the rabbis could as easily have made a narrow exemption in Jewish law for Jewish household servants in the Papal States.
Edgardo Mortara grew up a Catholic and was ordained a priest.
Kidnapping
The Church does not support kidnapping. Kidnapping is the illegal abduction of a person. The Church denounces kidnapping in the Catechism of the Catholic Church:
Kidnapping and hostage taking bring on a reign of terror; by means of threats they subject their victims to intolerable pressures. They are morally wrong (CCC 2297).
Even in the case of Edgardo Mortara, he was not kidnapped from his parents. Because his parents were unable and unwilling to raise him as a Christian, he was legally removed from their home based on the laws of that time and place. It would be analogous to a modern state choosing to remove a child from a home where it was determined that the parents were not fit to raise him. We might argue with whether or not the state made an accurate determination of parental fitness, but we could not justly accuse the state of kidnapping.
Edgardo’s Baptism
In the case of Edgardo Mortara, he was baptized by a Christian servant in his parents’ home when the servant believed he was dying. Because baptism is ordinarily necessary for salvation, canon law even today provides that a child in danger of death may be baptized without his parents’ consent:
An infant of Catholic parents, indeed even of non-Catholic parents, may in danger of death be baptized even if the parents are opposed to it (canon 868 §2, Code of Canon Law).
When Edgardo survived his illness, it became known that the child had been baptized. By law of the papal states, which was the secular authority of the place in Italy in which the Mortara family lived at the time, a baptized child could not be raised by unbaptized people, even his own parents. The concern was that a Christian be raised to believe and practice as a Christian, something unbaptized parents or guardians could not do. In the Mortara case, his parents were told that the child could be returned if they converted to Catholicism, but they chose not to do so.
Then and Now
In the nineteenth century, churchmen were working under a more restricted understanding of sacramental and pastoral theology. They believed sincerely that a baptized child who was not raised Christian was in serious danger of eternal damnation. Their actions in the Mortara case reflected that understanding. They were doing the best they could with the knowledge that they had to protect an innocent child from going to hell. Today, sacramental and pastoral theology has developed (through the Development of Doctrine) to the point where, with a child in those circumstances today, the Church would not advocate removing the child from his parents. We now understand that so long as the child and his parents acted rightly according to the truth that they understood, we can trust that God will take that into account and act mercifully in their regard.
Today, the parents would simply be notified of the baptism. If the family intended to always remain Jewish the matter would go no further. If they or their child eventually chose to enter the Church, it would be known that, in the child’s case, a conditional baptism for the sake of establishing a sacramental record would need to be performed.