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 begins his comments on Psalm 126 by noting that we sinful humans have given ourselves over into bondage to sin, and cannot redeem ourselves from that slavery (Augustine Psalms, loc. 340564, par. 1). Verse one speaks of the redeemed people of Zion receiving the comfort of the Lord. To understand Zion, or Jerusalem, as being both captive and the eternal heavenly city, Augustine sees the earthly Jerusalem as a shadowy representative of the heavenly one (Augustine Psalms, loc. 340569, par. 2). In verse two, our mouth is filled with joy. Augustine sees the "mouth" as a representative of the heart, which the Lord fills with joy (Augustine Psalms, loc. 340584, par. 3).
Because the heart is to be full of joy, it is necessary to guard the heart from evil (Augustine Psalms, loc. 340594, par. 5). This is done, as we understand from verse three, by remembering what the Lord has done for us. Even for those in sin, the Lord has done good (Augustine Psalms, loc. 340600, par. 6). His work is described in verse four as akin to a work of melting ice, reating a torrent of water (Augustine Psalms, loc. 340604, par. 7).
God's work of mercy brings a joyful harvest, even after sorrow in the time of planting (Augustine Psalms, loc. 340609, par. 8). We do works of mercy, and we look forward to a harvest. Augustine describes several ways in which Christians can plant their good works, and the corresponding harvests.