God has had pity (mercy) on us, and is presently having pity on us, by means of the Church and the Sacraments; He is blessing us. God teaches us through St. Augustine that, when He blesses us, we grow . Hence, because we have received His blessing of Baptism, we must grow. He not only has pity on us; He therefore gives us life and causes us to mature into the image of His Son. This is God letting His "face shed its light upon us." His face is His image, into which we are created when we are baptized, in order that we may do good works, as He does good works; the purpose being: "So will your ways be known upon earth and all nations learn your saving help." However, we must never forget: We cannot do anything without Him, even after Baptism. This is the Invitatory, inviting the Church to pray, even though we have already been baptized. We cannot say that we have received everything in Baptism and no longer need to pray. Because we are baptized, we must pray continuously, as our Lord Jesus did.
As a result of His mercy and blessing of His Church, He "let[s] the nations be glad and exult, for you rule the world with justice; with fairness you rule the peoples, you guide the nations on earth." No one can be "glad and exult" unless God grants it. We rejoice because God rules with justice and fairness. Justice and fairness encompass mercy and judgment. God is most merciful and pours His blessings out upon His Church; therefore, His chastisements of us is included in His blessings, e.g. His servant Job.
"Let the peoples praise you, O God; let all the peoples praise you." Because the Father and the Son has sent us the Holy Spirit, when we submit and walk in obedience, being the image of the Son, we praise God. He blesses us and His blessings are returning to Him as fruit is what we are taught by St. Augustine .
"The earth has yielded its fruit for God, our God, has blessed us. May God still give us his blessing till the ends of the earth revere him."
 www.divineoffice.org  St. Augustine (2010-03-28). St. Augustine: Exposition on the Book of Psalms (Kindle Locations 13797-13801). Kindle Edition.