1 We have already, beloved, as the Lord was pleased to enable us, expounded to you those words of the Gospel, where the Lord, in washing His disciples' feet, says, “He that is once washed needs not save to wash his feet, but is clean every whit.” Let us now look at what follows. “And ye,” He says, “are clean, but not all.” And to remove the need of inquiry on our part, the evangelist has himself explained its meaning, by adding: “For He knew who it was that should betray Him; therefore said He, You are not all clean.” Can anything be clearer? Let us therefore pass to what follows.
Source: Tractates on the Gospel of John (New Advent)