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.
As Psalm 149:1 calls us to sing a new song to the Lord, Augustine says a new song comes from the new man, who has been made new by God in Christ (Augustine Psalms, loc. 343286, par. 1). Augustine illustrates briefly that the new song shows joy and a life of charity. This is the new song of the Lord's field, where God's Word brings increase (Augustine Psalms, loc. 343301, par. 2). Verse two then calls all Israel to rejoice in God. Augustine considers all the Church to be Israel as we can see God (Augustine Psalms, loc. 343307, par. 3). This we do as we love God and our neighbor, which Augustine refers to as the "two wings of love."
Verse three further develops the idea of singing praise by calling for praise in chorus, a gathering (Augustine Psalms, loc. 343322, par. 4). Augustine emphasizes the harmony that Christians have in Christ, drawing their praises together.
Verse four begins with a reason for praise, that the Lord has dealt kindly with His people. Augustine immediately references the greatest kindness, that of Christ dying in our place (Augustine Psalms, loc. 343342, par. 5). Because of God's kindness, verse five speaks of the saints' exulting in glory. Augustine carefully distinguishes between the glory of fools and the glory of the wise and godly (Augustine Psalms, loc. 343347, par. 6). In sharp contrast to those who glory in public spectacles, Augustine speaks of a Christian virtue of taking one's home as a matter of exulting (Augustine Psalms, loc. 343357, par. 7). This places the praise of God into our mouths (v. 6). The Psalmist calls this a sharp two-edged sword, which Augustine speaks of as having a temporal and an eternal edge (Augustine Psalms, loc. 343367, par. 8). The sword, in Augustine's view, is used by the Lord to divide the world into different groups as He pleases. Augustine elaborates at some length on the work of God and men, as they engage in conflict. Verses 8-9 speak of an outcome, that of binding and judging rulers (Augustine Psalms, loc. 343391, par. 10). Augustine takes this to be obscure since some rulers are Christians. However, some of those are removed from authority and others limit their authority and act in a humble way. They are bound and humbled.