[quote="DavidFilmer, post:2, topic:320815"]
This was a highly contentious topic in the earliest days of the Church. In those days, there were a number of heresies, and people unwittingly joined heretical sects and were Baptized in them. When the heresy was exposed, those people wished to unite with the Catholic Church. Should the Church accept their Baptism?
The Early Fathers were sharply divided over this issue. My favorite ECF, St. Cyprian of Carthage, was perhaps the most vocal and effective opponent of the idea of accepting heretical Baptism. He would not even use the word "baptize" in this context - he referred to "those made wet by heretics." I think that's funny.
How can an unbaptized person perform valid Baptism? How can someone give what they do not have?
Alas for St. Cyprian, the Church determined that Baptism, as the gateway of salvation (and the only assured means of obtaining it) is not encumbered by the ontological nature of the minister. The Grace of Baptism comes from God, and God may pour out his Grace in any manner he chooses. The Church determined, by her divine authority to teach, that Baptismal Grace is made available even when the minister is not Baptized. Once this decision was made, opposition to it ceased.
So, basically the answer to your question is: Because the Church said so.
FWIW, in the Sacrament of Matrimony (in Latin theology), the bride and groom mutually confer the Sacrament upon each other. The deacon/priest/bishop witnesses the union and pronounces the blessing of the Church, but he is not considered to be the minister of the Sacrament. So Baptism is not unique in this regard.
Excellent answer! Thank you very much.