2. De Spiritu Sancto. The three books on the Holy Spirit may be considered as a continuation of the above treatise, and were also addressed to Gratian in compliance with his request, a.d. 381. In this treatise St. Ambrose shows that the Holy Spirit is God, and of one nature and substance with the Father and the Son. He makes use of the Greek writers, SS. Didymus, Basil the Great, and Athanasius, and was on this ground attacked by St. Jerome. See Rufinus, Apol. adv. Hieron. II. 23–25.