8 For if the sanctity of baptism be according to the diversity of merits in them that administer it, then as merits are diverse there will be diverse baptisms; and the recipient will imagine that what he receives is so much the better, the better he appears to be from whom he received it. The saints themselves— understand brethren, they that belong to the dove, that have their part in that city of Jerusalem, the good themselves in the Church, of whom the apostle says, “The Lord knows them that are His” — are endued with different graces, and do not all possess like merits.
Some are more holy than others, some are better than others. Therefore if one receive baptism from him, for example, who is a righteous saint, another from another who is of inferior merit with God, of inferior degree, of inferior continence, of inferior life, how notwithstanding is that which they receive one, equal and like, if it be not because, “This is He that baptizes”? Just, then, as when the good and the better administer baptism, one man does not receive a good thing, another a better; but, notwithstanding that the ministers were one good the other better, they receive what is one and equal, not a better in the one case and a worse in the other; so, too, when a bad man administers baptism, through the ignorance or forbearance of the Church (for bad men either are not known as such, or are borne with; the chaff is tolerated until the floor be fully purged at the last), that which is given is one, not unlike because the ministers are unlike, but like and equal because “This is He that baptizes.”
Source: Tractates on the Gospel of John (New Advent)