Isaiah 49:13 Sing, O heavens, and be joyful, O earth; and break forth into singing, O mountains: for the Lord hath comforted his people, and will have mercy upon his afflicted.
The heavens sing: the angels sang Glory to God and peace on earth when announcing the birth of the Savior of the world. There will be more joy in heaven over one sinner who repents. Every time a sinner repents, which happens daily in our lives, heaven is singing and rejoicing.
The earth is joyful: Joy to the world, the Lord has come. When God entered human history the gloomy and despairing earth rejoiced and was exceeding glad. Jesus was born and brought joy and peace to all who would believe in Him.
The mountains break forth into singing: He makes Lebanon skip like a calf and, and Sirion like a young wild ox. The creation itself is eagerly awaiting the redemption of the sons of God. We don’t see the angels of heaven rejoicing; we don’t normally see the joy of the earth; and we don’t hear the mountains singing for joy. But it happens.
Every time we repent and believe; every day we hear the Gospel and believe it; every week the Lord ministers the Gospel to us; every year we celebrate joyful remembrances like Christmas and Easter. When we do God rejoices and holds a party along with the visible and invisible creation. Repentance and faith makes God happy, for He loves us and He is happy when we return to Him daily. He is happy, of course, when we do good works and kind things, but that is because it is the evidence of repentance and faith.
Joy comes because the Lord comforts His people and has mercy on the afflicted. This promise is for us, for we are His people and we are afflicted. God made us a people through the redemption that is in Christ Jesus. Comfort comes daily in the midst of fear and worry. Affliction happens. It happens to all human beings because we are all sinners and live in a world of sin. Sin causes affliction of all kinds: physical, mental, emotional, relational, earthly, financial, societal, and much evil. Without mercy we will die; and God has mercy to the afflicted. Trust in and rest upon the mercy of God.