14 Pelagianus of Luperciana said: "It is written, 'Either the Lord is God, or Baal is God.' So now either the Church is the Church, or heresy is the Church. Further, if heresy be not the Church, how can the baptism of the Church exist among heretics?"
15. To him we may answer as follows: Either Paradise is Paradise, or Egypt is Paradise. Further, if Egypt be not Paradise, how can the water of Paradise be in Egypt? But it will be said to us that it extends even there by flowing forth from Paradise. In like manner, therefore, baptism extends to heretics. Also we say: Either the rock is the Church, or the sand is the Church. Further, since the sand is not the Church, how can baptism exist with those who build upon the sand by hearing the words of Christ and doing them not? And yet it does exist with them; and in like manner also it exists among the heretics.
Source: On Baptism, Against the Donatists (New Advent)