6 You have the Lord saying of the Pharisees, “They sit in Moses' seat.” The Lord did not mean them only; as if He would send those who should believe in Christ to the school of the Jews, that they might learn there wherein is the way to the kingdom of heaven. Did not the Lord come for this end, that He might establish a Church, and separate those Jews who had a good faith, and a good hope, and a good love, as wheat from the chaff, and might make them one wall of the circumcision, to which should be joined another wall from the uncircumcision of the Gentiles, of which two walls coming from different directions, Himself should be the Corner-Stone?
Did not the same Lord therefore say of these two people who were to be one, “And other sheep I have, which are not of this fold”? Now He was speaking to the Jews; “Them also,” said He, “must I bring, that there may be one fold, and One Shepherd.” Therefore there were two ships out of which He had called His disciples. They figured these two people, when they let down their nets, and took up so great a draught and so large a number of fishes, that the nets were almost broken.
“And they laded,” it is said, “both the ships.” The two ships figured the One Church, but made out of two peoples, joined together in Christ, though coming from different parts. Of this too the two wives, who had one husband Jacob, Leah and Rachel, are a figure. Of these two, the two blind men also are a figure, who sat by the way side, to whom the Lord gave sight. And if you pay attention to the Scriptures, you will find the two Churches, which are not two but One, figured out in many places.
For to this end the Corner-Stone serves, for to make of two One. To this end serves That Shepherd, for to make of two flocks One. So then the Lord who was to teach the Church, and to have a school of His Own beyond the Jews, as we see at present, would He be likely to send those who believe in Him unto the Jews, to learn? But under the name of the Scribes and Pharisees He intimated that there would be some in His Church who would say and not do; but, in the person of Moses He designated Himself.
For Moses represented Him, and for this reason did he put a veil before him, when he was speaking to the people; because as long as they were in the law given up to carnal joys and pleasures, and looking for an earthly kingdom, a veil was put upon their face, that they should not see Christ in the Scriptures. For when the veil was taken away, after that the Lord had suffered, the secrets of the temple were discovered. Accordingly when He was hanging on the Cross, the veil of the temple was rent from the top even to the bottom; and the Apostle Paul says expressly, “But when you shall turn to Christ, the veil shall be taken away.” Whereas with him who turns not to Christ, though he read the law of Moses, the veil is laid upon his heart, as the Apostle says.
When the Lord then would signify beforehand that there would be some such in His Church, what did He say? “The Scribes and Pharisees sit in Moses' seat. What they say, do; but do not what they do.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)