Theophylact of Ohrid

Or, because many think that they worship God in the spirit, i. e. with the mind, who yet held heretical doctrines concerning Him, for this reason He adds, and in truth. May not the words too refer to the two kinds of philosophy among us, i. e. active and contemplative; the spirit standing for action, according to the Apostle, As many as are led by the Spirit of God; (Rom. 8:14) truth, on the other hand, for contemplation. Or, (to take another view,) as the Samaritans thought that God was confined to a certain place, and ought to be worshipped in that place; in opposition to this notion, our Lord may mean to teach them here, that the true worshippers worship not locally, but spiritually. Or again, all being a type and shadow in the Jewish system, the meaning may be that the true worshippers will worship not in type, but in truth. God being a Spirit, seeketh for spiritual worshippers; being the truth, for true ones.