If Celsus, indeed, had understood our teaching regarding the Spirit of God, and had known that “as many as are led by the Spirit of God, these are the sons of God,” he would not have returned to himself the answer which he represents as coming from us, that “God put His own Spirit into a body, and sent it down to us;” for God is perpetually bestowing of His own Spirit to those who are capable of receiving it, although it is not by way of division and separation that He dwells in (the hearts of) the deserving.
Nor is the Spirit, in our opinion, a “body,” any more than fire is a “body,” which God is said to be in the passage, “Our God is a consuming fire.” For all these are figurative expressions, employed to denote the nature of “intelligent beings” by means of familiar and corporeal terms. In the same way, too, if sins are called “wood, and straw, and stubble,” we shall not maintain that sins are corporeal; and if blessings are termed “gold, and silver, and precious stones,” we shall not maintain that blessings are “corporeal;” so also, if God be said to be a fire that consumes wood, and straw, and stubble, and all substance of sin, we shall not understand Him to be a “body,” so neither do we understand Him to be a body if He should be called “fire.”
In this way, if God be called “spirit,” we do not mean that He is a “body.” For it is the custom of Scripture to give to “intelligent beings” the names of “spirits” and “spiritual things,” by way of distinction from those which are the objects of “sense;” as when Paul says, “But our sufficiency is of God; who has also made us able ministers of the New Testament; not of the letter, but of the spirit; for the letter kills, but the spirit gives life,” where by the “letter” he means that “exposition of Scripture which is apparent to the senses,” while by the “spirit” that which is the object of the “understanding.”
It is the same, too, with the expression, “God is a Spirit.” And because the prescriptions of the law were obeyed both by Samaritans and Jews in a corporeal and literal manner, our Saviour said to the Samaritan woman, “The hour is coming, when neither in Jerusalem, nor in this mountain, shall you worship the Father. God is a Spirit; and they that worship Him must worship Him in spirit and in truth.” And by these words He taught men that God must be worshipped not in the flesh, and with fleshly sacrifices, but in the spirit.
And He will be understood to be a Spirit in proportion as the worship rendered to Him is rendered in spirit, and with understanding. It is not, however, with images that we are to worship the Father, but “in truth,” which “came by Jesus Christ,” after the giving of the law by Moses. For when we turn to the Lord (and the Lord is a Spirit), He takes away the veil which lies upon the heart when Moses is read.
Source: Against Celsus (New Advent)