84 Clarus of Mascula said: "The sentence of our Lord Jesus Christ is manifest, when He sent forth His apostles, and gave the power which had been given Him of His Father to them alone, whose successors we are, governing the Church of the Lord with the same power, and baptizing those who believe the faith. And therefore heretics, who, being without, have neither power nor the Church of Christ, cannot baptize any one with His baptism."
85. Are, then, ill-affectioned murderers successors of the apostles? Why, then, do they baptize? Is it because they are not outside? But they are outside the rock, to which the Lord gave the keys, and on which He said that He would build His Church.
Source: On Baptism, Against the Donatists (New Advent)