1 In this Psalm, the humility of one that is a servant of God and faithful is commended unto us, by whose voice it is sung; which is the whole body of Christ. For we have often warned you, beloved, that it ought not to be received as the voice of one man singing, but of all who are in Christ's Body. And since all are in His Body, as it were one man speaks: and he is one who also is many....Now he prays in the temple of God, who prays in the peace of the Church, in the unity of Christ's Body; which Body of Christ consists of many who believe in the whole world: and therefore he who prays in the temple, is heard. For he prays in the spirit and in truth, who prays in the peace of the Church; not in that temple, wherein was the figure....
2. “Lord, my heart is not lifted up”. He has offered a sacrifice. Whence do we prove that he has offered a sacrifice? Because humility of heart is a sacrifice....If there is no sacrifice, there is no Priest. But if we have a High Priest in Heaven, who intercedes with the Father for us (for He has entered into the Holy of Holies, within the veil),...we are safe, for we have a Priest; let us offer our sacrifice there. Let us consider what sacrifice we ought to offer; for God is not pleased with burnt-offerings, as you have heard in the Psalm. But in that place he next shows what he offers: The sacrifice of God is a troubled spirit: a broken and a contrite heart, O God, shall Thou not despise.
3. “Lord, my heart was not lifted up, neither were my eyes raised on high”; “I have not exercised myself in great matters, nor in wonderful things which are too high for me”. Let this be more plainly spoken and heard. I have not been proud: I have not wished to be known among men as for wondrous powers; nor have I sought anything beyond my strength, whereby I might boast myself among the ignorant. As that Simon the sorcerer wished to advance into wonders above himself, on that account the power of the Apostles more pleased him, than the righteousness of Christians....What is above my strength, he says, I have not sought; I have not stretched myself out there, I have not chosen to be magnified there. How deeply this self-exaltation in the abundance of graces is to be feared, that no man may pride himself in the gift of God, but may rather preserve humility, and may do what is written: “The greater you are, the more humble yourself, and you shall find favour before the Lord:” how deeply pride in God's gift should be feared, we must again and again impress upon you....
4. “If I had not lowly thoughts, but have lifted up my soul, as one taken from his mother's breast, such the reward for my soul”. He seems as it were to have bound himself by a curse:...as though he had been going to say, Let it so happen to me. “As one taken away from his mother's breast, may be my soul's reward.” You know that the Apostle says to some weak brethren, “I have fed you with milk, and not with meat; for hitherto you were not able to bear it, neither yet now are you able.” There are weak persons who are not fit for strong meat; they wish to grasp at that which they cannot receive: and if they ever do receive, or seem to themselves to receive what they have not received, they are puffed up thereby, and become proud thereupon; they seem to themselves wise men. Now this happens to all heretics; who since they were animal and carnal, by defending their depraved opinions, which they could not see to be false, were shut out of the Catholic Church....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)