God is a Spirit. This is a confirmation drawn from the very nature of God. Since men are flesh, we ought not to wonder, if they take delight in those things which correspond to their own disposition. Hence it arises, that they contrive many things in the worship of God which are full of display, but have no solidity. But they ought first of all to consider that they have to do with God, who can no more agree with the flesh than fire with water. This single consideration, when the inquiry relates to the worship of God, ought to be sufficient for restraining the wantonness of our mind, that God is so far from being like us, that those things which please us most are the objects of his loathing and abhorrence. And if hypocrites are so blinded by their own pride, that they are not afraid to subject God to their opinion, or rather to their unlawful desires, let us know that this modesty does not hold the lowest place in the true worship of God, to regard with suspicion whatever is gratifying according to the flesh. Besides, as we cannot ascend to the height of God, let us remember that we ought to seek from His word the rule by which we are governed. This passage is frequently quoted by the Fathers against the Arians, to prove the Divinity of the Holy Spirit, but it is improper to strain it for such a purpose; for Christ simply declares here that his Father is of a spiritual nature, and, therefore, is not moved by frivolous matters, as men, through the lightness and unsteadiness of their character, are wont to be.

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.

God is a Spirit - This is one of the first, the greatest, the most sublime, and necessary truths in the compass of nature! There is a God, the cause of all things - the fountain of all perfection - without parts or dimensions, for he is Eternal - filling the heavens and the earth - pervading, governing, and upholding all things: for he is an infinite Spirit! This God can be pleased only with that which resembles himself: therefore he must hate sin and sinfulness; and can delight in those only who are made partakers of his own Divine nature. As all creatures were made by him, so all owe him obedience and reverence; but, to be acceptable to this infinite Spirit, the worship must be of a spiritual nature - must spring from the heart, through the influence of the Holy Ghost: and it must be in Truth, not only in sincerity, but performed according to that Divine revelation which he has given men of himself. A man worships God in spirit, when, under the influence of the Holy Ghost, he brings all his affections, appetites, and desires to the throne of God; and he worships him in truth, when every purpose and passion of his heart, and when every act of his religious worship, is guided and regulated by the word of God. "The enlightened part of mankind," says Abu'l Fazel, "knows that true righteousness is an upright heart; and believe that God can only be worshipped in holiness of Spirit." Ayeen Akbery, vol. iii. p. 254.
"Of all worshippers," says Creeshna, "I respect him as the most devout, who hath faith in me, and who serveth me with a soul possessed of my spirit." Geeta, p. 68.

God [is] a (h) Spirit: and they that worship him must worship [him] in spirit and in truth.
(h) By the word "spirit" he means the nature of the Godhead, and not the third person in the Trinity.

God is a spirit,.... Or "the Spirit is God"; a divine person, possessed of all divine perfections, as appears from his names, works, and worship ascribed unto him; See Gill on John 4:23; though the Arabic and Persic versions, and others, read as we do, "God is a spirit"; that is, God, Father, Son, and Holy Ghost: for taking the words in this light, not one of the persons is to be understood exclusive of the other; for this description, or definition, agrees with each of them, and they are all the object of worship, and to be worshipped in a true and spiritual manner. God is a spirit, and not a body, or a corporeal substance: the nature and essence of God is like a spirit, simple and uncompounded, not made up of parts; nor is it divisible; nor does it admit of any change and alteration. God, as a spirit, is immaterial, immortal, invisible, and an intelligent, willing, and active being; but differs from other spirits, in that he is not created, but an immense and infinite spirit, and an eternal one, which has neither beginning nor end: he is therefore a spirit by way of eminency, as well as effectively, he being the author and former of all spirits: whatever excellence is in them, must be ascribed to God in the highest manner; and whatever is imperfect in them, must be removed from him:
and they that worship him; worship is due to him on account of his nature and perfections, both internal and external; with both the bodies and souls of men; and both private and public; in the closet, in the family, and in the church of God; as prayer, praise, attendance on the word and ordinances:
must worship him in spirit and in truth; in the true and spiritual manner before described, which is suitable to his nature, and agreeably to his will.

God is a Spirit. Rather, "God is Spirit." This declaration is fundamental. Since he is Spirit, he must receive spiritual worship, and is everywhere present.

God is a Spirit - Not only remote from the body, and all the properties of it, but likewise full of all spiritual perfections, power, wisdom, love, holiness. And our worship should be suitable to his nature. We should worship him with the truly spiritual worship of faith, love, and holiness, animating all our tempers, thoughts, words, and actions.