The LORD is . . . the horn of my salvation.
In Scripture the horn often denotes royal power as the chief power, or the king himself as the chief, especially the victorious and warlike power. Therefore also the Church is called the horn of Christ, Psalm 112:9: "His horn will be exalted in glory"; and Psalm 75:10: "The horns of the righteous shall be exalted," that is, the kingdoms and churches of Christ. . .. Thus Luke 1:69 reads: "He has raised up a horn of salvation," that is, Christ as victorious salvation, "for us in the house of His servant David." ... . But the kingdom of Christ is called His horn. Accordingly, He Himself has one horn, or one realm, as it were, because He is the Lord of only one Church. For just as a horned beast . . . fights and wages war with the horns as weapons, so Christ fights and vanquishes the world and its prince through the Church, which is His exceedingly strong and invincible horn. The Church, on the contrary, calls Christ her horn, because it also denotes the King Himself, as I have said, because Christ is the strength of His Church through whom it triumphs over the world. He Himself is strength and wisdom for all who believe in Him. Thus the avenging God is the strength and invincible might of Christ, in which He has overcome all things. Accordingly, He says here, "the horn of My salvation," for God is the victorious might of Christ for salvation.
From First Lectures on the Psalms, on Psalm 18 (Luther's Works 10:113)