12 At length, hear who it is that asks drink: “Jesus answered and said unto her, If you knew the gift of God, and who it is that says to you, Give me to drink, you would, it may be, have asked of Him, and He would have given you living water.” He asks to drink, and promises to give drink. He longs as one about to receive; He abounds as one about to satisfy. “If you knew,” says He, “the gift of God.” The gift of God is the Holy Spirit. But as yet He speaks to the woman guardedly, and enters into her heart by degrees.
It may be He is now teaching her. For what can be sweeter and kinder than that exhortation? “If you knew the gift of God,” etc.: thus far He keeps her in suspense. That is commonly called living water which issues from a spring: that which is collected from rain in pools and cisterns is not called living water. And it may have flowed from a spring; yet if it should stand collected in some place, not admitting to it that from which it flowed, but, with the course interrupted, separated, as it were, from the channel of the fountain, it is not called “living water:” but that is called living water which is taken as it flows. Such water there was in that fountain. Why, then, did He promise to give that which He was asking?
Source: Tractates on the Gospel of John (New Advent)