The three books on the Holy Spirit are, as St. Ambrose says himself, a sequel to those on the Faith, and the two treatises together have been sometimes quoted as if one, with the title, De Trinitate. But we see from Gratian's letter to St. Ambrose, and from the reply, that each treatise is separate, and the De Spiritu Sancto was written some years later, a.d. 381.
In the first book St. Ambrose commences by allegorizing the history of Gideon and the fleece, seeing in the drying of the fleece and the moistening of the threshing-floor a type of the Holy Spirit leaving the Jews and being poured out on the Gentiles. Passing to his more immediate subject, he proves that the Holy Spirit is above the whole Creation and is truly God, alleging as a special argument that the sin against the Holy Spirit can never be forgiven, here or hereafter. He shows how the Holy Spirit is in Scripture called the Spirit of God; that He spoke by the prophets and apostles; that He sanctifies men, and is typified by the mystical ointment spoken of in Scripture. Next, St. Ambrose treats of His oneness with the other two Persons of the Holy Trinity, and shows that His mission in no way detracts from this oneness, but that there is in all the Divine Persons a perfect unity of peace, love, and other virtues.
The second book commences with a treatment of the history of Samson in the same way as that of Gideon in Book I. Samson always succeeded so long as the Holy Spirit was with him, but fell into misfortune so soon as he was forsaken. It is shown that the power of the Holy Spirit is the same as that of the Father and the Son, and that there is an agreement in design and working, and in vivifying man. He is Creator and therefore to be worshipped, and He worked with the Father and the Son in founding the Church, and in conclusion is proved the unity of operation in the Three Persons.
The third book continues the same argument, showing that the mission of prophets and apostles, and even of the Son Himself, is to be referred to the Spirit, yet without any subjection on the part of the Son, seeing that the Spirit also receives His mission from the Father and the Son. The Godhead of the Holy Spirit is next taken up and proved, when occasion is taken also to show that there are not three Gods or three Lords, for the Three Divine Persons are one in holiness and nature; and the work is concluded with a summary of some of the principal arguments.
There can be but little doubt that this is the work, and St. Ambrose the author, bitterly attacked by St. Jerome; the whole passage may be read in the Apology of Rufinus, p. 470, in vol. iii. of this series. St. Ambrose is compared to a daw decked in another bird's plumage, and charged with writing “bad things in Latin taken from good things in Greek,” and St. Jerome even took the trouble to translate a work of St. Didymus on the Holy Spirit (from the preface to which the above extracts are taken), in order that those who did not know Greek might, St. Jerome hoped, recognize the plagiarisms.
Rufinus vigorously defends St. Ambrose, and, pointing out many inconsistencies in his opponent, says: “The saintly Ambrose wrote his book on the Holy Spirit not in words only but with his own blood, for he offered his life-blood to his persecutors, and shed it within himself, though God preserved his life for future labours.”
The truth is that St. Ambrose being a good Greek scholar, and having undertaken to write on the Holy Spirit, studied what others had written before him, and made use of what had been urged by SS. Basil, Didymus, and others. The opinion of the great St. Augustine concerning this treatise may be set against that of St. Jerome. “St. Ambrose when treating of the deep subject of the Holy Spirit, and showing that He is equal with the Father and the Son, yet makes use of a simple style of discourse; inasmuch as his subject required no the embellishments of language, but proofs to move the minds of his readers.”
Source: On the Holy Spirit (New Advent)