Although less renowned than City of God or Confessions, The Trinity is arguably Augustine’s Magnum Opus. During the fourth century, one of the longest and most widespread theological controversies centered on the doctrine of the Trinity, culminating in the Council of Constantinople in 381 AD. Most experts believe he penned this during 400-416 AD following this period of intense debate.
For more of Augustine, check out the Fathers of the Church: St. Augustine (30 vols.).
“For this reason it is difficult to contemplate and to comprehend fully the substance of God, which makes changeable things without any change in itself, and creates temporal things without any temporal movement of its own. Therefore, the purification of our soul is necessary in order that it may be able to see that ineffable thing in an ineffable manner. Since we have not as yet become endowed with this, we are strengthened by faith and are led along more accessible roads, in order that we may gain the proficiency and skill to grasp that reality.” (Pages 5–6)
“Therefore, let everyone who reads these pages proceed further with me, where he is as equally certain as I am; let him make inquiries with me where he is as equally hesitant as I am; wherever he recognizes the error as his, let him return to me; wherever it is mine, let him call me back. Thus let us enter together on the path of charity in search of Him of whom it is said: ‘Seek his face evermore.’1 This is the sacred and safe compact into which I, in the presence of the Lord our God, shall enter with those who read what I am writing, in all my writings, and especially in the present one where we are investigating the unity of the Trinity, of the Father, the Son, and the Holy Spirit. For nowhere else is the error more dangerous, the search more laborious, and the results more rewarding.” (Page 8)
“I ask, then, of whom is he speaking in another place: ‘For from him and through him, and in him are all things. To him be the glory forever. Amen’? For if, of the Father, the Son, and the Holy Spirit so that separate actions are attributed to separate persons, that is, ‘from him,’ from the Father, ‘through him,’ through the Son, and ‘in him,’ in the Holy Spirit, it is self-evident that the Father, the Son, and the Holy Spirit are one God when he added in the singular number: ‘To him be the glory forever. Amen.’” (Page 16)
Aurelius Augustinus (354–430) is often simply referred to as St. Augustine or Augustine Bishop of Hippo (the ancient name of the modern city of Annaba in Algeria). He is the preeminent Doctor of the Church according to Roman Catholicism, and is considered by Evangelical Protestants to be in the tradition of the Apostle Paul as the theological fountainhead of the Reformation teaching on salvation and grace.