60 Tenax of Horrea Celiæ said: "There is one baptism, but of the Church; and where the Church is not, there baptism also cannot be."
61. To him we answer: How then comes it that it may be where the rock is not, but only sand; seeing that the Church is on the rock, and not on sand?
Source: On Baptism, Against the Donatists (New Advent)