John 4:24

The AV has Jesus saying, "God is a spirit." One could infer that He is one spirit among many. The NASB and NIV have, "God is spirit." The Greek text has no indefinite article ("a"), but it is legitimate to supply one, as is often true in similar anarthrous (without the article) constructions. However the absence of the article often deliberately stresses the character to the noun (cf. 1 John 1:5; 1 John 4:8). That seems to have been Jesus’ intention here.

The sense of the passage is that God is spirit as opposed to flesh. He is invisible, divine, and essentially unknowable. Nevertheless He has chosen to reveal Himself (John 1:1-18). Since He is a spiritual rather than a corporeal being, those who worship Him must do so in a spiritual rather than a material way. A spiritual birth (John 3:5) is prerequisite for spiritual worship.

The essential reason worship of God must be spiritual is that God is a spiritual being, not a physical idol. Worship of a spiritual God requires spiritual worship, not just going through certain acts of worship at special places of worship. Furthermore, people cannot worship God in any manner that may seem attractive to them. They must worship Him as He by the Spirit has revealed we should.