As, then, by the higher mystical ascent from matters that concern ourselves to that transcendent nature we gain a knowledge of the Word, by the same method we shall be led on to a conception of the Spirit, by observing in our own nature certain shadows and resemblances of His ineffable power. Now in us the spirit (or breath) is the drawing of the air, a matter other than ourselves, inhaled and breathed out for the necessary sustainment of the body. This, on the occasion of uttering the word, becomes an utterance which expresses in itself the meaning of the word.
And in the case of the Divine nature it has been deemed a point of our religion that there is a Spirit of God, just as it has been allowed that there is a Word of God, because of the inconsistency of the Word of God being deficient as compared with our word, if, while this word of ours is contemplated in connection with spirit, that other Word were to be believed to be quite unconnected with spirit. Not indeed that it is a thought proper to entertain of Deity, that after the manner of our breath something foreign from without flows into God, and in Him becomes the Spirit; but when we think of God's Word we do not deem the Word to be something unsubstantial, nor the result of instruction, nor an utterance of the voice, nor what after being uttered passes away, nor what is subject to any other condition such as those which are observed in our word, but to be essentially self-subsisting, with a faculty of will ever-working, all-powerful.
The like doctrine have we received as to God's Spirit; we regard it as that which goes with the Word and manifests its energy, and not as a mere effluence of the breath; for by such a conception the grandeur of the Divine power would be reduced and humiliated, that is, if the Spirit that is in it were supposed to resemble ours. But we conceive of it as an essential power, regarded as self-centred in its own proper person, yet equally incapable of being separated from God in Whom it is, or from the Word of God whom it accompanies, as from melting into nothingness; but as being, after the likeness of God's Word, existing as a person, able to will, self-moved, efficient, ever choosing the good, and for its every purpose having its power concurrent with its will.
Source: The Great Catechism (New Advent)