An intriguing turn of phrase:
...the covenant relation of Yahweh to Israel is a covenant of marital love, which---as in Hosea's magnificent vision---moves and stirs Yahweh himself to his heart. He has loved the young maiden Israel with a love that has proved to be indestructible, eternal. He can be angry with the wife of his youth on account of her adultery. He can punish her, but all this is simultaneously directed against himself and pains him, the lover, whose "bowels churn." He cannot repudiate her without rendering judgment against himself. It is on this, on his personal, innermost bewilderment as lover, that the covenant's eternal and irrevocable character is based.
Emphasis mine. God cannot do something without rendering judgment against himself? This is, then, what covenant means: a promise so unbreakable, it acts to restrain Omnipotence Himself. That single sentence in its context was for me a whole day's worth of pondering.
To place it more in context, after Ratzinger has used the book of Hosea as above to establish the significance of covenant, he goes on to show the part Mary plays as Israel's representative:
...the relationship between God and Israel includes not only God but Israel as woman... for this reason the covenant, which forms the very basis of the existence of Israel as a nation and the existence of each individual as an Israelite, is expressed interpersonally in the fidelity of the marriage covenant and no other way....But above all, this also means that to God, the One, is joined, not a goddess, but, as in his historical revelation, the chosen creature, Israel, the daughter Zion, the woman...
In the women of Israel, the mothers [e.g., Sarah, Rachel, Hannah] and saviors [e.g., Esther, Judith, Deborah], in their fruitful infertility is expressed most purely and most profoundly what creation is and what election is, what "Israel" is as God's people....
Emphasis mine again. "Fruitful infertility" is an expression of creation (something from nothing) and election (saving the un-saveable). Again, there is something to chew on for a long time.
Of course, this line of development...acquires its definitive meaning for the first time in the New Testament: in the woman who is herself described as the true holy remnant, as the authentic daughter Zion...
He is setting up Mary as a type, as a representative or personification of Israel-as-woman, indeed of all creation as woman (or Woman), the one whose life is the perfect response.
Incidentally, on the next page Ratzinger begins to discuss the feminine figure of Wisdom in the Old Testament, and offers this striking descriptive definition of Wisdom (Sophia) that is no doubt intended to be thematically linked to the previous:
"Wisdom" appears ... as God's first creature in whom both the pure, primordial form of his creative will and the pure answer, which he discovers, find their expression; indeed one can say that precisely this concept of the answer is formative for the Old Testament idea of wisdom. Creation answers, and the answer is as close to God as a playmate, as a lover.
All this is in the space of a couple of pages.