9 The introduction of a pause marks a change of person. He no longer speaks but is addressed. For now the prophetic utterance assumes a general character. Thus immediately after the prayer addressed to God, he has added, in order that the confidence of the speaker might be understood to have obtained what He was asking even in the very moment of asking: Behold, God is My helper and the Lord is the upholder of My soul. He has requited evil unto Mine enemies.
To each separate petition he has assigned its proper result, thus teaching us both that God does not neglect to hear, and that to look for a pledge of His pitifulness in hearing our several petitions is not a thing unreasonable. For to the words, For strangers are risen up against Me, the corresponding statement is: God is My helper; while with regard to and the violent have sought after My soul, the exact result of the hearing of His prayer is expressed in the words: and the Lord is the upholder of My soul; lastly the statement, they have not set God before their eyes, is appropriately balanced by, He has requited evil unto Mine enemies.
Thus God both gives help against those that rise up, and upholds the soul of His Holy One when it is sought by the violent, and when He is not set before the eyes, nor considered by the ungodly, He requites upon His enemies the very evils which they had wrought; so that while without thinking upon God they seek the soul of the righteous and rise up against Him, He is saved and upheld, and they find that He Whom, absorbed in their wicked works, they did not consider, avenges their malice by turning it against themselves.
Source: Homilies on the Psalms (New Advent)