God is spirit, and his worshipers must worship in spirit and in truth.
(John 4:24, NIV)
To worship God in spirit and in truth means to give Him the worship that is His due.
God is a spirit. Therefore He must be worshiped in spirit, by a humble and genuine worship in the depths of our souls, where He alone can see it.
As we worship Him more and more often, this worship becomes a natural response to His continual presence with us. God becomes one with our souls and our souls one with Him. What began as our sporadic response of love gradually blends into a unison of love with Him. We begin to taste the beautiful, unbroken union Christ enjoys with the Father.