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.
Psalm 90 is called "The prayer of Moses the Man of God" (Augustine Psalms, loc. 334550, par. 1). Augustine interprets it, along with all the teaching of Moses, as an example for us, who find our refuge in God (v. 1). Verse two speaks of the pre-existent nature of God, present before the world existed. Augustine extends this to apply to all creation, including the angels (Augustine Psalms, loc. 334570, par. 3). In saying this, Augustine stands against all the forms of Gnosticism which assert God to be part of creation.
In light of the eternity and exalted nature of God, in verse three, the prayer is that man would not be turned aside "to lowness." Augustine ties this to all our many prayers that we would be one with God, not led to temptation (Augustine Psalms, loc. 334583, par. 4). The God who is greater than time is the one to whom we look. Augustine emphasizes that the eternal God not only views a thousand years as yesterday, but all times, even the future, as a recent occurrence (Augustine Psalms, loc. 334596, par. 5). In comparison, even a long human life is but a fleeting moment.
Verse seven makes it clear that our short human life is a penalty for our sin. Though God sustains and carries us, we are still frail (Augustine Psalms, loc. 33610, par. 7). All our sin is known to God (v. 7) and we come to an end, in earthly terms, within about 70-80 years (vv. 9-10). Because human ages vary so much, though, Augustine seeks some spiritual or allegorical reason for this statement (Augustine Psalms, loc. 334624, par. 9). One possible explanation is the cutting short of life that happened at the time of the Flood. Another he suggests is that the numbers 15 and 150 seem to have a spiritual significance, and 150 is the total of the 70 or 80 years we have.
The Psalm speaks to the shortness of our life, when in verse 11 it refers to God's power and anger. Augustine is clear that God's power and wrath lead us to ask for mercy, as well as to pursue good works (Augustine Psalms, loc. 334650, par. 11). For this reason the Psam speaks of instruction guiding God's people.
Even in our pursuit of the good, we become weary from striving with evil. Therefore, in verse 13, we ask the Lord to return. Augustine does recall that we are not looking for a physical motion on the part of God, but for him to look on us with a merciful attitude (Augustine Psalms, loc. 334682, par. 14). Verse 14 anticipates God's mercy as well. In the merciful gaze of God we have a confidence. God knows all our time (Augustine Psalms, loc. 334702, par. 15). Regardless of the trials we endure, we are God's works, preserved by His mercy (vv. 16-17). Augustine sees this as yet another reason for hope. Our days are known by God (Augustine Psalms, loc. 334723, par. 17).