Augustine of Hippo
Augustine of Hippo is arguably the most influential Christian theologian since Paul. Born in North Africa to a pagan father and a Christian mother (Monica), he spent his youth in restless intellectual and moral searching, moving through Manichaeism and Neo-Platonism before his dramatic conversion in Milan in 386 AD, described in his Confessions — perhaps the most celebrated autobiography ever written. As Bishop of Hippo from 396 AD until his death, he wrote extensively against Donatism, Pelagianism (defending the doctrines of original sin and sovereign grace), and paganism (City of God). His theology of grace, predestination, original sin, the Trinity, and the Church defined Western Christianity for a thousand years and directly shaped both Roman Catholic and Protestant theology at the Reformation.
On God and the Restless Heart
Thou madest us for Thyself, and our heart is restless, until it repose in Thee.
Confessions, I.1.1 (NPNF 1.1) (Ps. 62:1; Eccl. 3:11)
Late have I loved Thee, O Beauty so ancient and so new; late have I loved Thee! For behold Thou wert within me, and I outside; and I sought Thee outside and in my unloveliness fell upon those lovely things that Thou hast made. Thou wert with me and I was not with Thee.
Confessions, X.27.38 (NPNF 1.1) (Ps. 16:11; Acts 17:27–28)
Our heart is restless until it rests in you...You stir man to take pleasure in praising You, because You have made us for Yourself, and our heart is restless until it finds its rest in You.
Confessions, I.1.1 (NPNF 1.1) (Ps. 73:25–28; Matt. 5:6)
On Grace and Original Sin
...no man comes to Christ unless the Father draw him. Let no man say, "I came of myself"; let no man glory as if he came by his own act. You did not choose God; you were chosen by God.
Tractates on John, 26.6 (NPNF 1.7) (John 6:44; John 15:16; Eph. 1:4)
If they were asked whether grace is given according to merits, they would answer, No; because if it were so, it would not be grace at all...What could be more clear than this, — that grace is not given according to merits, but according to God's free gift?
On Nature and Grace, 36 (NPNF 1.5) (Rom. 11:6; Eph. 2:8–9)
For it is He Who, of His own will, causes us to will what is good; it is He Who works in us both to will and to do of His good pleasure. And this He does not by forcing, but by converting the will.
On Grace and Free Will, 16.32 (NPNF 1.5) (Phil. 2:13; Ezek. 36:27)
On Scripture
Confine not your children to your own learning, for they were born in another time than yours. There is one book of which the contents are always fresh and new — and where can we be better nourished than in the pastures of Holy Scripture?
Letter 189 (NPNF 1.1) (2 Tim. 3:16–17; Ps. 119:105)
If we are perplexed by an apparent contradiction in Scripture, it is not allowable to say, The author of this book is mistaken; but either the manuscript is faulty, or the translation is wrong, or you have not understood.
Letter 82.3 (NPNF 1.1) (John 10:35; 2 Sam. 23:2)
On the Trinity
And so the Father is God, and the Son is God, and the Holy Spirit is God; and yet they are not three Gods, but one God: in which Trinity none is before or after another, none is greater or less than another; but the whole three persons are co-eternal together, and co-equal.
On the Trinity, I.2.4 (NPNF 1.3) (Matt. 28:19; 1 Cor. 8:6; John 10:30)
The Father is not the Son, and the Son is not the Father, and the Holy Spirit...is neither the Father nor the Son. Nevertheless they are not three Gods, but one God; nor do they work separately, for their operation is inseparable.
On the Trinity, IV.21.30 (NPNF 1.3) (Deut. 6:4; John 14:16–17)
On the Two Cities
Two cities have been formed by two loves: the earthly by the love of self, even to the contempt of God; the heavenly by the love of God, even to the contempt of self. The former, in a word, glories in itself, the latter in the Lord.
City of God, XIV.28 (NPNF 1.2) (Rom. 13:14; Matt. 22:37–39; Phil. 3:19–20)
Accordingly, the peace of the celestial city is the perfectly ordered and harmonious enjoyment of God, and of one another in God. The peace of all things is the tranquillity of order.
City of God, XIX.17 (NPNF 1.2) (Rev. 21:1–4; Isa. 9:6–7)