1 You already well know, dearest brethren, that a “Song of Degrees,” is a song of our ascent: and that this ascent is not effected by the feet of the body, but by the affections of the heart. This we have repeatedly reminded you of: and we need not repeat it too often, that there may be room for saying what has not yet been said. This Psalm, therefore, which you have now heard sung for you, is inscribed, “A Song of Degrees.” This is its title. They sing therefore while ascending: and sometimes as it were one man sings, sometimes as it were many; because many are one, since Christ is One, and in Christ the members of Christ constitute one with Christ, and the Head of all these members is in heaven. But although the body toils on earth, it is not cut off from its Head; for the Head looks down from above, and regards the body....Whether therefore one or many sing; many men are one man, because it is unity; and Christ, as we have said, is One, and all Christians are members of Christ.
2....Certain members indeed of that body of which we also are, which can sing in truth, have gone before us. And this the holy Martyrs have sung: for they have already escaped, and are with Christ in joy about to receive at last incorruptible bodies, the very same which were at first corruptible, wherein they have suffered pains; of the same there will be made for them ornaments of righteousness. Therefore whether they in reality, or we in hope, joining our affections with their crowns, and longing for such a life as we have not here, and shall never gain unless we have longed for it here, let us all sing together, and say, “If the Lord Himself had not been in us.”...
3. “If the Lord Himself had not been in us, now may Israel say”...When? “When men rose up against us”. Marvel not: they have been subdued: for they were men; but the Lord was in us, man was not in us: for men rose up against us. Nevertheless men would crush other men, unless in those men who could not be crushed, there were not man, but the Lord. For what could men do to you, while you rejoiced, and sang, and securely held everlasting bliss? What could men do to you when they rose against you, if the Lord had not been on your side? What could they do? “Perchance they had swallowed us up quick”. “Swallowed us up:” they would not first have slain us, and so have swallowed us up. O inhuman, O cruel men! The Church swallows not thus. To Peter it was said, “Kill and eat:” not, Swallow quick. Because no man enters into the body of the Church, save he be slain first. What he was dies, that he may be what he was not. Otherwise, he who is not slain, and is not eaten by the Church, may be in the visible number of the people: but he cannot be in the number of the people which is known to God, whereof the Apostle says, “The Lord knows who are His,” save he be eaten; and eaten he cannot be, save he first be slain. The Pagan comes, still in him idolatry lives; he must be grafted among the members of Christ: that he may be engrafted, he must needs be eaten; but he cannot be eaten by the Church, save first he be slain. Let him renounce the world, then is he slain; let him believe in God, then is he eaten...But they in whom the Lord is, are slain and die not. But they who consent and live, are swallowed quick, when swallowed up they die. But they who have suffered, and have not yielded to tribulations, rejoice and say, “If the Lord had not been in us,” etc.
4....“When their fury was enraged upon us.” They are now in anger, they now openly rage: “perchance the water had drowned us”. By water he means ungodly nations: and we shall see what sort of water in the following verses. Whoever had consented unto them, water would have overwhelmed him. For he would die by the death of the Egyptians, he would not pass through after the example of the Israelites. For you know, brethren, that the people of Israel passed through the water, by which the Egyptians were overwhelmed. But what sort of water is this? It is a torrent, it flows with violence, but it will pass by...Hence He, our Head, first drinks, of whom it is said in the Psalms, “He shall drink of the torrent in the way: therefore shall He lift up His head.” For our Head is already exalted, because He drank of the torrent by the way; for our Lord has suffered. If therefore our Head has been already raised up, why does the body fear the torrent? Without doubt, because the Head has been raised, the body also will say hereafter, “Our soul has passed over the torrent. Perhaps our soul has passed over the water without substance”. Behold, what sort of water he was speaking of, “The water perchance had overwhelmed us.” But what means, “without substance”?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)