40 Julianus of Telepte said: "It is written, 'A man can receive nothing, except it be given him from heaven.' if heresy is from heaven, it can give baptism."
41. Let him hear another also saying: If covetousness is from heaven, it can give baptism. And yet the covetous do confer it; so therefore also may the heretics.
Source: On Baptism, Against the Donatists (New Advent)