Augustine of Hippo — on Rev 1:4 (The City of God, Book 17)

“"For the barren," she says, "hath born seven, and she that hath many children is waxed feeble." Here all that had been prophesied hath shone forth to those who understood the number seven, which signifies the perfection of the universal Church, For which reason also the Apostle John writes to the seven churches, showing in that way that he writes to the totality of the one Church; and in the Proverbs of Solomon it is said aforetime, prefiguring this, "Wisdom hath builded her house, she hath strengthened her seven pillars." For the city of God was barren in all nations before that child arose whom we see.”