God is a Spirit: and they that worship him must worship in spirit and truth.
Barnes’ Notes on the Bible
God is A spirit – This is the second reason why men should worship him in spirit and in truth. By this is meant that God is without a body; that he is not material or composed of parts; that he is invisible, in every place, pure and holy. This is one of the first truths of religion, and one of the sublimest ever presented to the mind of man. Almost all nations have had some idea of God as gross or material, but the Bible declares that he is a pure spirit. As he is such a spirit, he dwells not in temples made with hands Acts 7:48, neither is worshipped with men’s hands as though he needed anything, seeing he giveth to all life, and breath, and all things, Acts 17:25. A pure, a holy, a spiritual worship, therefore, is such as he seeks – the offering of the soul rather than the formal offering of the body – the homage of the heart rather than that of the lips.