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.
Augustine takes Psalm 89 to be intended to strengthen us, who in the world, are weak by nature (Augustine Psalms, loc. 334126, par. 1). The Psalm calls out to God, singing His mercies. Verse two finds God saying "mercy shall be built up for ever" (Augustine Psalms, loc. 334132, par. 3). In this verse, Augustine notes, mercy and truth are closely related. The truth Augustine finds emphasized is that Jesus, the cornerstone, is the savior for Israelites (one wall) and Gentiles (another wall) (Augustine Psalms, loc. 334146, par. 3). Verse three goes on to establish that God's covenant, which He made, is wit hHis chosen. God has sworn an oath, which is a promise. This continues in verse four, where Augustine is clear that God's covenant is with David's seed, Christ, who makes all who believe heirs of the promise (Augustine Psalms, loc. 334159, par. 5).
Verse five declares that even the heavens praise God's work. Augustine illustrates how in all the good we see, we recognize God's merciful works (Augustine Psalms, loc. 334179, par. 6). Verse six continues that in all creation there is none comparable to God. He is "to be had in dread of all them that are round about Him" (v. 7) (Augustine Psalms, loc. 324205, par. 8). Augustine quickly notes that God, being limitless, is difficult to conceive of as being surrounded. However, in the Incarnation, there is a legitimate way to picture a local presence. This continues even in the resurrection, as Jesus does possess a literal body.
There is turmoil in God's presence. In verse nine He rules the sea. Augustine sees this as an indicator that God uses such forces as the buffeting of the waves to scatter people around the world, bearing the Gospel with them (Augustine Psalms, loc. 334232, par. 10). Through verse 12 the power of God over all nature is emphasized. The nations do recognize the power of God and rejoice in His name, which Augustine sees as the name of light (Augustine Psalms, loc. 334252, par. 13). God is consistently pictured as the one who is might and able to judge rightly (vv. 13-14). Augustine is clear that the righteous judgment and mercy of God are what make us willing to come into God's presence (Augustine Psalms, loc. 334265, par. 15). This also provides the motivation found in verse 15. The people can shout before God because they know His mercy. Again, in verse 17, God's praise is due to His mercy, which was His idea, coming from His pleasure. Verse 18, then, sees God as the one who gathers His people (Augustine Psalms, loc. 334285, par. 18).
By verse 20 the focus shifts back to God's personal care for His people, as He has anointed David as His servant. Augustine recalls for us in this context, through the verses which follow, that Jesus is the fulfillment of David's kingdom (Augustine Psalms, loc. 334299, par. 20ff). In his work, none of his enemies will overcome him, but in the end he will defeat his enemies. Eventually, in verse 25, God will even extend this to "the sea." Augustine takes this as a reference to the entiles (Augustine Psalms, loc. 334313, par. 25). The covenant with Jesus, the Son, bears everlasting mercy (vv. 28-29). Augustine understands the mercy of God in Christ to be poured out on all Christians (Augustine Psalms, loc. 334333, par. 29). Yet he also sees the mercy of God shown as He chastizes His people (vv. 30-32). As a sign of God's mercy, though, the chastizement is poured out on Christ, God the Son, rather than on the Christian. Augustine unpacks this idea at some length (Augustine Psalms, loc. 334353, par. 3). In the end, God will not deny His covenant (v. 34). He remains the God who reaises up a people to receive the promise to Abraham (Augustine Psalms, loc. 334373, par. 31). Verse 35 describes this as the security of God's people, that God always keeps His promises. Augustine considers the promise which is of greatest value as a proof of God's character to be the bodily resurrection. He also sees the resurrection as the article of the faith that people dispute, to their own harm (Augustine Psalms, loc. 334406, par. 32).
The reality of God's mercy is denied by the people of the Psalm, just as by people in Augustine's time. Verses 38-40 speak of the rejection (Augustine Psalms, loc. 334412, par. 33). Augustine continues to emphasize that the Psalm is fulfilled in Christ. He concludes that David also knew that he himself was not the fulfillment of the prophecy (Augustine Psalms, loc. 334438, par. 34).
What happens to the enemies of God? In verses 42 and following they are turned away in battle. God has vengeance on His enemies (Augustine Psalms, loc. 334458, par. 34). Apart from Christ, God's promises are not fulfilled. Augustine finds this as the great barrier between God and man (Augustine Psalms, loc. 334464, par. 34). Yet God does fulfill His promises. In verses 46 and following, the Psalmist again asks that God would not hide His mercy. Again, Augustine finds this mercy brought to life in Christ, who rose from the dead, fulfilling verse 48 (Augustine Psalms, loc. 334484, par. 37). The prayer, in verse 50, becomes that the Lord would remember his people in their hardship. Augustine recalls the type of hardships which fall on the Church (Augustine Psalms, loc. 334511, par. 39). Even in these troubles the Christian has hope. Augustine closes with reminders that God is able to rescue and gather His people.