The phrase comes from the wonderful story in John 4 about Jesus’ encounter with a Samaritan woman at a well Jacob had dug centuries earlier. Jesus asks the woman for a drink, yet she questions why He would, since Jews typically avoided Samaritans.
Jesus replies that He is the source of living water, saying,
“The water that I shall give him will become in him a fountain of water springing up into everlasting life.”
He is declaring to her, I am the Messiah. I am the Promised One.
The woman states,
“Sir, I perceive that You are a prophet. Our fathers worshiped on this mountain, and you Jews say that in Jerusalem is the place where one ought to worship.”
She’s essentially asking, Who is right? Jesus responds by saying quite clearly,
“Salvation is of the Jews.”
Then He adds:
“But the hour is coming, and now is, when the true worshipers will worship the Father in spirit and truth; for the Father is seeking such to worship Him. God is Spirit, and those who worship Him must worship in spirit and truth.”
So, what is a clear definition of “spirit and in truth”?
Jesus tells us in John 14:6,
“I am the way, the truth, and the life. No one comes to the Father except through Me.”
If we want to worship the Father in truth, we must worship through Jesus, His only Begotten Son—sent into the world to reconcile us to God through His precious blood.
Yet we must also worship in the Spirit, freely given to us by Jesus, who is the baptizer in the Spirit. As John the Baptist testifies in John 1:33:
“‘This is He who baptizes with the Holy Spirit.’ And I have seen and testified that this is the Son of God.”
So to worship in spirit and in truth is to worship the Father in the Holy Spirit with the salvation that Jesus provides. That is what God is looking for.