36 Donatus of Cibaliana said: "I acknowledge one Church, and one baptism that appertains thereto. If there is any one who says that the grace of baptism exists among heretics, he must first show and prove that the Church exists with them."
37. To him we answer: If you say that the grace of baptism is identical with baptism, then it exists among heretics; but if baptism is the sacrament or outward sign of grace, while the grace itself is the abolition of sins, then the grace of baptism does not exist with heretics. But so there is one baptism and one Church, just as there is one faith. As therefore the good and bad, not having one hope, can yet have one baptism, so those who have not one common Church can have one common baptism.
Source: On Baptism, Against the Donatists (New Advent)