Aurelius Ambrosius, better known in English as Saint Ambrose (c. 340 - 4 April 397), was an archbishop of Milan who became one of the most influential ecclesiastical figures of the 4th century. He was one of the four original doctors of the Church. He is patron saint of Milan.
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.