The name of Christ is used here instead of the Church, because the similitude was intended to apply—not to God's only-begotten Son, but to us. It is … - John Calvin

This is the highest honour of the Church, that, until He is united to us, the Son of God reckons himself in some measure imperfect. What consolation is it for us to learn, that, not until we are along with him, does he possess all his parts, or wish to be regarded as complete! Hence, in the First Epistle to the Corinthians, when the apostle discusses largely the metaphor of a human body, he includes under the single name of Christ the whole Church.

Our feeling of ignorance, vanity, want, weakness, in short, depravity and corruption, reminds us ... that in the Lord, and none but He, dwell the true light of wisdom, solid virtue, exuberant goodness. We are accordingly urged by our own evil things to consider the good things of God; and, indeed, we cannot aspire to Him in earnest until we have begun to be displeased with ourselves. For what man is not disposed to rest in himself? Who, in fact, does not thus rest, so long as he is unknown to himself; that is, so long as he is contented with his own endowments, and unconscious or unmindful of his misery? Every person, therefore, on coming to the knowledge of himself, is not only urged to seek God, but is also led as by the hand to find him.

If there had been any unbelief in Mary, that could not prevent God from accomplishing his work in any other way which he might choose. But she is called blessed, because she received by faith the blessing offered to her, and opened up the way to God for its accomplishment.