Popes Leo and Gregory (the Greats)
In other of his Marian writings, Pope St. Leo praises Christ’s miraculous and “immaculate birth” from the Virgin Mary and said “Christ is born from the body of his unsullied Mother” with an analogy to our baptism where we are “born again” (John 3:3,5; Titus 3:5) and become members of the Church (1 Cor 12:12-13; Acts 2:38; Gal 3:26-27; Eph 4:4-5; etc):
“For we would have been unable to overcome the author of sin and death had Christ not assumed our nature and made it his own. Sin cannot defile him, nor can death hold him. For he was conceived by the Holy Spirit within the womb of a Virgin Mother, who gave birth to him without losing her virginity, just as she had remained a virgin in conceiving him.” (Leo I, Epist 28:2; PL 54:759; Gambero, page 304-5 or 306)
“He foretold to the serpent that the woman’s seed would come and crush his haughty and wicked head with his power (cf. Gen 3:15). The woman’s seed is Christ, who was to come in the flesh as God and as man, born of the Virgin, to condemn the despoiler of the human race by his immaculate birth.” (Leo I, Sermo 22:1; PL 54:194; Gambero, page 307)
“By the Spirit, Christ is born from the body of his unsullied Mother; by this same Spirit, the Christian is reborn from the womb of holy Church.” (Leo I, Sermo 29:1; PL 54:227; Gambero, page 308)
Pope St. Gregory exalts the Virgin Mary as higher than the highest mountains, with several biblical analogies:
"The most blessed and ever-Virgin Mary, Mother of God, can be called by this name, ‘mountain’. Yes, she was a mountain, who by the dignity of her election has completely surpassed the height of every elect creature. Is Mary not a lofty mountain? For God, to achieve the conception of the eternal Word, raised the summit of her merits above the choirs of angels, up to the threshold of the Godhead. Isaiah said in a prophecy, ‘In the last days, the mountain of the Lord’s house will be made the highest mountain’ (Isa 2:2). And this mountain has been made the highest mountain, because Mary’s height has shined out above all the saints. For, just as a mountain implies height, so the house signifies a dwelling place. Therefore she is called mountain and house, because she, illuminated by incomparable merits, prepared a holy womb for God’s Only-begotten to dwell in.
"On the other hand, Mary would not have become a mountain raised above the peaks of the mountains had not the divine fecundity raised her above the angels. Further, she should not have become the Lord’s house had not the divinity of the Word assumed humanity and come to dwell in her womb. Mary is justly called a mountain rich in fruits, because the best fruit was born from her, namely, a new man. And the prophet, considering how beautiful she is, adorned in the glory of her fruitfulness, cries out: ‘There shall come forth a shoot from the stump of Jesse, and a branch shall grow from his roots’ (Isa 11:1).
"David, exulting in the fruits of this mountain, says to God, ‘Let the peoples praise you, O Lord, let all the peoples praise you. The earth has yielded its fruit’ (Psalm 67:6-7). Yes, the earth has yielded its fruit, because the Virgin did not conceive her Son by man’s doing but because the Holy Spirit stretched out his shadow over her. Therefore the Lord says to David, king and prophet, ‘I will place the fruit of your womb upon your throne’ (Psalm 132:11).
So says Isaiah, ‘And the fruit of the earth shall be exalted’ (Isa 4:2). For him whom the Virgin bore was not only a holy man but also the mighty God. Elizabeth refers to this fruit when she greets the Virgin and says, ‘Blessed are you among women, and blessed is the fruit of your womb’ (Luke 1:42). Mary is rightly called mountain of Ephrem because, while she is raised up by the ineffable dignity of the divine birth, the dry branches of the human condition flower again in the fruit of her womb." (Gregory I [the Great], In I Regum 1:5; PL 79:25-26; Gambero, page 371-2)
Also, by Pope Leo (I) the Great’s time in the west, we have the explicit statements of St. Augustine and St. Ambrose on the matter:
“With the exception, therefore, of the holy Virgin Mary, in whose case, out of respect for the Lord, I would have no question raised when there is talk of sin – for how do we know what further grace was conferred on her for absolute victory over sin, she who deserved to conceive and bear Him who obviously had no sin? – with the exception, then, of this Virgin, could we but gather together in their lifetime all those saints, men and women, and ask them whether they were free from sin, what in our opinion would have been their answer? …No matter how remarkable their holiness in this body…they would have cried out with one voice: ‘If we should say that we have no sin, we deceive ourselves, and the truth is not in us’ [1 John 1:8].” (Augustine, De natura et gratia, cap 36, n. 42)
“Come, then, and search out Your sheep, not through Your servants or hired men, but do it Yourself. Life me up bodily and in the flesh, which is fallen in Adam. Lift me up not from Sara but from Mary, a Virgin not only undefiled but a Virgin whom grace has made inviolate, free of every stain of sin [Latin: ut incorrupta sit virgo, sed virgo per gratiam ab omni integra labe peccati].” (Ambrose, Commentary on Psalm 118, Jurgens, Faith of the Early Fathers, vol 2, page 166)
The Virgin Mary is an exception to “all have sinned…” (Rom 3:23; 1 John 1:8; etc) according to St. Augustine and St. Ambrose. The debate in later centuries was over whether Mary was “cleansed” from birth, or immaculate from conception. However, everyone knew she was sinless or “fully graced” from birth and throughout her life.
The Immaculate Conception of the Mother God from Juniper Carol