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 observes as a note of encouragement that Psalm 100 is not very long. He then moves to the content of verse one, where the Psalm calls all the lands to sing joyfully to the Lord (Augustine Psalms, loc. 336268, par. 3). He sees this as a natural reaction to the work of the Gospel in every nation, as it brings good. However, Augustine also questions the concept in light of the Psalm's title. It is a psalm of confession (Augustine Psalms, loc. 336273, par. 3).
Augustine concludes that jubilation is not necessarily something that can be expressed in words and with reason. We don't always understand our joy or the reason and extent of it (Augustine Psalms, loc. 336284, par. 4).
Jubilation, in a sense, happens "when we praise that which cannot be uttered" (Augustine Psalms, loc. 336289, par. 5). Augustine describes a whole universe of order which cannot be explained but which causes jubilation. He even sees that God himself contemplates the creation with joy. Therefore our jubilation participates in the joy of the Lord (Augustine Psalms, loc. 336303, par. 6).
The jubilation we engage in is a way of serving the Lord. Augustine observes that servitude is disappointing by nature. Yet serving God does not lead to discontent, but to joy (Augustine Psalms, loc. 336313, par. 7) Augustine goes on to consider the monastic life, as a place where numerous like-minded people serve God. This can be a haven of rest and peace, but the storms will still come and can cause shipwreck (Augustine Psalms, loc. 336329, par. 8). The storms come because all those in monastic life have sin in themselves. Even as we wish to love God, there will at least be some who are of a wicked intent (Augustine Psalms, loc. 336344, par. 9). Christians find themselves in sin just like those who are not Christians.
Verse two calls all the lands to serve God with gladness. Augustine contrasts this to "bitterness of murmering" (Augustine Psalms, loc. 336363, par. 10). We come to God with joy. Verse three explains that we know joy because we know the Lord is God. He is the one who made us. He therefore understands us. We find joy because we can depend on Him (Augustine Psalms, loc. 336373, par. 11). So, also in verse three, our reaction is confession (Augustine Psalms, loc. 336373, par. 12).
Verse four speaks of the Lord as "pleasant." Augustine finds this to be for our strengthening. Knowing that the Lord is pleasant urges us to feed on his goodness (Augustine Psalms, loc. 336389, par. 13).
When verse five says God's truth is "from generation to generation" Augustine takes this in one of two ways. Either God's truth is long lasting or it applies both to the earthly and the heavenly realms (Augustine Psalms, loc. 336394, par. 13). Augustine makes no clear conclusion of this interpretation.