St. Athanasius: "that...men might be enabled to know Him"

“For God, being good and loving to mankind, and caring for the souls made by Him, ─since He is by nature invisible and incomprehensible, having His being beyond all created existence, for which reason the race of mankind was likely to miss the way to the knowledge of Him, since they are mad out of nothing while He is unmade, ─for this cause God by His own Word gave the Universe the Order it has, in order that since He is by nature invisible, men might be enabled to know Him at any rate by His works.”

~St. Athanasius: Against the Heathen, Chap. 35.

Lactantius: "There are two ways"

"THERE are two ways, O Emperor Constantine, by which human life must proceed—the one which leads to heaven, the other which sinks to hell; and these ways poets have introduced in their poems, and philosophers in their disputations. And indeed philosophers have represented the one as belonging to virtues, the other to vices; and they have represented that which belongs to virtues as steep and rugged at the first entrance, in which if any one, having overcome the difficulty, has climbed to the summit, they say that he afterwards has a level path, a bright and pleasant plain, and that he enjoys abundant and delightful fruits of his labours; but that those whom the difficulty of the first approach has deterred, glide and turn aside into the way of vices, which at its first entrance appears to be pleasant and much more beaten, but afterwards, when they have advanced in it a little further, that the appearance of its pleasantness is withdrawn, and that there arises a steep way, now rough with stones, now overspread with thorns, now interrupted by deep waters or violent with torrents, so that they must be in difficulty, hesitate, slip about, and fall. And all these things are brought forward that it may appear that there are very great labours in undertaking virtues, but that when they are gained there are the greatest advantages, and firm and incorruptible pleasures; but that vices ensnare the minds of men with certain natural blandishments, and lead them captivated by the appearance of empty pleasures to bitter griefs and miseries—an altogether wise discussion, if they knew the forms and limits of the virtues themselves. For they had not learned either what they are, or what reward awaits them from God: but this we will show in these two books."

~Lactantius (c. 250 – c. 325): The Divine Institutes, Bk. VI, Ch. 6.

Christopher H. Dawson on St. Augustine of Hippo

"IT was in this age of ruin and distress that St. Augustine lived and worked. To the materialist, nothing could be more futile than the spectacle of Augustine busying himself with the reunion of the African Church and the refutation of the Pelagians, while civilisation was falling to pieces about his ears. It would seem like the activity of an ant which works on while its nest is being destroyed. But St. Augustine saw things otherwise. To him the ruin of civilisation and the destruction of the Empire were not very important things. He looked beyond the aimless and bloody chaos of history to the world of eternal realities from which the world of sense derives all the significance which it possesses. His thoughts were fixed, not on the fate of the city of Rome or the city of Hippo, nor on the struggle of Roman and barbarian, but on those other cities which have their foundations in heaven and in hell, and on the warfare between ‘the world-rulers of the dark aeon’ and the princes of light. And, in fact, though the age of St. Augustine ended in ruin and though the Church of Africa, in the service of which he spent his life, was destined to be blotted out as completely as if it had never been, he was justified in his faith. The spirit of Augustine continued to live and bear fruit long after Christian Africa had ceased to exist. It entered into the tradition of the Western Church and moulded the thought of Western Christendom so that our very civilisation bears that imprint of his genius. However far we have travelled since the fifth century and however much we have learnt from other teachers, the work of St. Augustine remains an inalienable part of our spiritual heritage."

~Christopher Dawson: Enquiries into Religion and Culture.

St. Cyril of Alexandria: Mother of God

“HAIL, from us, Mary, Mother of God, majestic common-treasure of the whole world, the lamp unquenchable, the crown of virginity, the staff of orthodoxy, the indissoluble temple, the dwelling of the Illimitable, Mother and Virgin, through whom He in the holy Gospels is called the ‘Blessed Who cometh in the name of the Lord.’”

~St. Cyril of Alexandria (c. 376 – 444 AD): Homilies, 4.

“IF anyone does not confess that Emmanuel is true God, and that therefore the Holy Virgin is Mother of God (Dei genetricem—Theotokon), since she bore, after the flesh, the incarnate Word of God, let him be anathema.”

~Council of Alexandria: Anathema 1 of St. Cyril. (430 AD)

Concerning the Devil

And I said to him, “I should like to continue my questions.” “Speak on,” said he. And I said, “I heard, sir, some teachers maintain that there is no other repentance than that which takes place, when we descended into the water and received remission of our former sins.” He said to me, “That was sound doctrine which you heard; for that is really the case. For he who has received remission of his sins ought not to sin any more, but to live in purity. Since, however, you inquire diligently into all things, I will point this also out to you, not as giving occasion for error to those who are to believe, or have lately believed, in the Lord. For those who have now believed, and those who are to believe, have not repentance for their sins; but they have remission of their previous sins. For to those who have been called before these days, the Lord has set repentance. For the Lord, knowing the heart, and foreknowing all things, knew the weakness of men and the manifold wiles of the devil, that he would inflict some evil on the servants of God, and would act wickedly towards them. The Lord, therefore, being merciful, has had mercy on the work of His hand, and has set repentance for them; and He has entrusted to me power over this repentance. And therefore I say to you, that if any one is tempted by the devil, and sins after that great and holy calling in which the Lord has called His people to everlasting life, he has opportunity to repent but once. But if he should sin frequently after this, and then repent, to such a man his repentance will be of no avail; for with difficulty will he live.” And I said, “Sir, I feel that life has come back to me in listening attentively to these commandments; for I know that I shall be saved, if in future I sin no more.” And he said, “You will be saved, you and all who keep these commandments.”

─Hermas: The Pastor (or "The Shepherd") Bk. II, Commandment 12, Ch. 3.
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I say to him, “Sir, listen to me for a moment.” “Say what you wish,” says he. “Man, sir,” say I, “is eager to keep the commandments of God, and there is no one who does not ask of the Lord that strength may be given him for these commandments, and that he may be subject to them; but the devil is hard, and holds sway over them.” “He cannot,” says he, “hold sway over the servants of God, who with all their heart place their hopes in Him. The devil can wrestle against these, overthrow them he cannot. If, then, you resist him, he will be conquered, and flee in disgrace from you. As many, therefore,” says he, “as are empty, fear the devil, as possessing power. When a man has filled very suitable jars with good wine, and a few among those jars are left empty, then he comes to the jars, and does not look at the full jars, for he knows that they are full; but he looks at the empty, being afraid lest they have become sour. For empty jars quickly become sour, and the goodness of the wine is gone. So also the devil goes to all the servants of God to try them. As many, then, as are full in the faith, resist him strongly, and he withdraws from them, having no way by which he might enter them. He goes, then, to the empty, and finding a way of entrance, into them, he produces in them whatever he wishes, and they become his servants.” 

─Hermas: The Pastor, Bk. II, Commandment 12, Ch. 5
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Regarding the devil and his angels, and the opposing influences, the teaching of the Church has laid down that these beings exist indeed; but what they are, or how they exist, it has not explained with sufficient clearness. This opinion, however, is held by most, that the devil was an angel, and that, having become an apostate, he induced as many of the angels as possible to fall away with himself, and these up to the present time are called his angels.

─Origen: On First Principles, Preface, ("De Principiis," Proem.) 6.
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The devil’s snare does not catch you, unless you are first caught by the devil’s bait.

─St. Ambrose: Explanation of Psalm 118, 14, 37.
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The devil rules over lovers of temporal goods belonging to this visible world, not because he is lord of this world, but because he is ruler of those covetous desires by which we long for all that passes away.

─St. Augustine: The Christian combat ("De Agone Christiano"), Bk. 1, Ch. 1.
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For there is more than one way in which men sacrifice to the fallen angels.

─St. Augustine: Confessions, Bk. 1, Ch. 1 

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Fall of the Rebel Angels, by Domenico Beccafumi.
Oil on wood, c. 1528; San Niccolò al Carmine, Siena.

St. Ephraem of Syria: "I have made thee a foundation of holy Church"

“SIMON, My disciple, I have made thee a foundation of holy Church, I called thee Rock on a previous occasion, because thou shalt be the support of all the buildings; thou art the overseer of those who will build My Church on earth. If they try to build anything evil, thou, the foundation, shalt reprimand them. Thou art the head of the fountain from which My doctrine shall be drunk, thou art the head of my disciples; I will give all people to drink through thee; yours is that life-giving sweetness I pour forth; I have chosen thee to be as it were the firstborn in My establishment and thou shalt become the heir of My treasures. The keys of My kingdom I have given thee. Lo, I have established thee prince over all My treasures.”

~St Ephraem of Syria: Sermons on Holy Week, 4, 1.

Christ Giving the Keys to St. Peter, Giovanni Battista Castello.
Illumination on vellum, 1598; Musée du Louvre, Paris.

St. John of Damascus: Annunciation of Our Lady

● “So then, after the assent of the holy Virgin, the Holy Spirit descended on her, according to the word of the Lord, which the angel spoke, purifying her and granting her power to receive the divinity of the Word, and likewise power to bring forth.”

─St. John of Damascus: Exposition of the Orthodox Faith, Book III, Chap. 2. 

● “Today is the crowning of our salvation, and the manifestation of the mystery which was from eternity. The Son of God becometh the Son of the Virgin, and Gabriel giveth the good tidings of grace. Therefore with him let us cry to the Mother of God, Hail! full of grace, the Lord is with thee.”

─Byzantine Menaea, Troparian for the Feast. (ca. 6th cent.)

Annunciation, by Alessandro Allori.
Oil on canvas, 1603; Galleria dell'Accademia, Florence.