Albert Barnes Commentary John 4:10

"Jesus answered and said unto unto her, If thou knewest the gift of God, and who it is that saith to thee, Give me to drink; thou wouldest have asked of him, and he would have given thee living water." — John 4:10 (ASV)

The gift of God. The word gift here denotes favor. It may refer to Jesus himself as the gift of God to the world, given to save humanity from death (John 3:16; 2 Corinthians 9:15), or it may refer to the opportunity then afforded her of seeking salvation.

If you knew what a favorable opportunity God now gives you to gain a knowledge of Himself, and so on. And who it is, and so on. If you knew that the Messiah was speaking.

Living water. The Jews used the expression living water to denote springs, fountains, or running streams, in opposition to dead and stagnant water. Jesus here means to denote by it His doctrine, or His grace and religion, in opposition to the impure and dead notions of the Jews and the Samaritans (John 4:14).

This was one of the many instances in which He took the opportunity from common topics of conversation to introduce religious discourse. None ever did it so aptly as He did, but by studying His example and manner, we may also learn to do it.

One way to acquire this art is to have the mind full of the subject: to make religion our first and main thing; to carry it with us into all employments and into all society; to look upon everything in a religious light, and out of the abundance of the heart the mouth will speak (Matthew 12:34).