Augustine. Exposition on the Book of Psalms. Schaff, Philip (editor). New York: Christian Literature Publishing Co, 1886. Nicene and Post-Nicene Church Fathers: Series 1: Volume VIII. Re-published 2014, Catholic Way Publishing. Kindle electronic edition, ISBN-13: 978-1-78379-372-3.
Mercy and judgment are bound together in Psalm 101:1. Augustine therefore cautions his reader against assuming there will never be any punishment because of God's mercy. God's judgment can be severe, even where there is mercy (Augustine Psalms, loc. 336401, par. 1). Augustine also asks whether mercy is first in the list because the present time is the time of mercy. The mercy of God may be used to call to repentance, while the unrepentant receive judgment (Augustine Psalms, loc. 336411, par. 1). Augustine does find a recurring pattern of mercy leading to repentance for those who beliefe, but to judgment for those who reject God's will.
The response to God's mercy, as expressed in verse two, is a singing with a harp (Augustine Psalms, loc. 336431, par. 3). This requires, as the Psalm says, walking in innocence. Augustine takes the innocent to be also merciful and readily pleased with good works. The one who is not innocent hates himself in one way or another (Augustine Psalms, loc. 336441, par. 3).
Verse three emphasizes the life of the innocent, who does not set anything evil up as an example (Augustine Psalms, loc. 336450, par. 4). The wicked sets himself up as just, so he is free to break God's law (Augustine Psalms, loc. 336461, par. 5). In contrast, in verse four, "when the wicked man departed from me, I knew him not" (Augustine Psalms, loc. 336466, par. 6). The righteous and the wicked are simply not in fellowship with one another.
The Psalmist continues in verse five even to say the righteous man will persecute the sins of the wicked (Augustine Psalms, loc. 336476, par. 7). It is not, to Augustine, a matter of avoiding the unrighteous altogether. However, the gatherings for meals and the attendant alliances are untrustworthy (Augustine Psalms, loc. 336496, par. 8).
In contrast, the faithful look to God (v. 6). The proud man is not included in God's house, but the humble are (Augustine Psalms, loc. 336507, par. 10). God's people are in the nature of God's house, showing mercy. Those who show no mercy will be scattered. Augustine concludes that the time for mercy is now (Augustine Psalms, loc. 336518, par. 12).