40 As for the reason why, though it is concealed among strangers whose wife the Church is, it is not hidden whose sister she is, it is plainly because it is obscure and hard to understand how the human soul and the Word of God are united or mingled, or whatever word may be used to express this connection between God and the creature. It is from this connection that Christ and the Church are called bridegroom and bride, or husband and wife. The other relationship, in which Christ and all the saints are brethren by divine grace and not by earthly consanguinity, or by the father and not by the mother, is more easily expressed in words, and more easily understood.
For the same grace makes all the saints to be also brethren of one another; while in their society no one is the bridegroom of all the rest. So also, notwithstanding the surpassing justice and wisdom of Christ, His manhood was much more plainly and readily recognized by strangers, who, indeed, were not wrong in believing Him to be man, but they did not understand His being God as well as man. Hence Jeremiah says: "He is both a man, and who shall know Him?" He is a man, for it is made manifest that He is a brother. And who shall know Him? For it is concealed that He is a husband. This must suffice as a defense of our father Abraham against Faustus' impudence and ignorance and malice.
Source: Reply to Faustus the Manichaean (New Advent)