The heavens declare the glory of God, and the sky above proclaims His handiwork.
The psalmist describes both the sun and the day, Christ and the Gospel, in a most delightful way when he says, "The heavens proclaim the glory of God" (Psalm 19 [:1]); that is, just as the natural heavens bring the sun and the day, and the sun is in the heavens, so the apostles in their preaching bring and have in themselves the true sun, Christ. The psalmist continues: "In them He has set a tent for the sun, which comes out like a bridegroom from his wedding chamber. It rejoices like a strong man to run its course. It rises from one end of the heavens, and runs back again to the same end, and there is nothing hidden from its heat" (Psalm 19 [:4-6]). All of this is said about the beautiful daybreak, that is, about the Gospel, which Scripture praises highly, for it makes alive, happy, joyous, active, and brings all goodness with it. Therefore, it is called "Gospel," that is, joyous news.
From the Church Postil, sermon for Advent 1 on Romans 13:11-14 (Luther's Works 75:18-19)