As much as I love St. Paul, and his second letter to the Corinthians in particular, I've always found today's first reading (2 Cor 1:18-22) a bit hard to grasp. It's easy enough to appreciate its poetry, but what does it mean to say "however many are the promises of God, their 'yes' is in him [Christ]"?
Today I think I got a bit of an insight.
Paul isn't saying that Christ was a "yes" to God's promises as in "Sure, I agree with them. Great promises!" He's saying that Christ is the "yes" as the one who fulfilled the promises; carried them out personally; took them on himself.
And that means that the "Amen from us through him to God for glory" is no mere, "Sure, I agree," either. It's the expression of our willingness to let the promises (and all they import) into our lives. In other words, Mary's "Be it done to me." Or as we say three times a day in the Our Father (morning prayer, Mass, evening prayer: you knew that, right?), "Thy Kingdom come."