John 4.23: But the hour is coming, and is now here, when the true worshipers will worship the Father in spirit and truth, for the Father is seeking such people to worship him. (ESV)
In verse 23 Jesus draws a distinction between true worship and by implication worship that is false. The nature of true worship is that it is in spirit and truth. Jesus has affirmed in John 14.6 that he is the truth, so the truth Jesus is connecting us to, in terms of worship and everything else, can be understood as everything he ever taught or demonstrated by his life. All of these teachings and actions are set forth for us in the four Gospel accounts. Furthermore, attributes of Jesus the Messiah, his nature, character, and mission are prophesied about and taught in the Bible as a whole. And so by learning from the teachings of Jesus and everything the Bible teaches us about him, and following him, we can thus worship in truth.
Such true worship is also in spirit. I believe this idea can be understood, as a mere starting point of understanding, that genuine worship doesn’t need to be outward or observable. The definition of the word for “spirit” Jesus uses here has the basic meaning of “breath, or life-spirit, or soul”. The usage of this word in Bible times was wide and varied. But the above definition indicates that true worship goes deep inside of us, like breathing air into us does. The English language derives the word “pneumonia” from the Greek word that Jesus uses in this verse.
Since worship begins in our innermost being, it is something that can be unobservable to other people. Worship in spirit may lead to outward expression, but it doesn’t necessarily have to. Since Jesus made a reference to worship in Jerusalem in verse 22, it seems as though there may be a tacit distinction being drawn between true worship in spirit and truth and the worship of the Pharisees in Jerusalem. They were widely known for their outward and pretentious worship practices designed to put on a good show.