The thoughts of 16th century Lutheran musicologist Lucas Lossius:
The Church celebrates the triple advent (or “coming”) of Christ. First is the advent into flesh, which is despised and humble before the world, of which Zechariah 9[:9] says, “Behold, Your King comes to you, gentle and poor, sitting upon a donkey, on a colt, the foal of a beast of burden,” [cf.] Matthew 21[:5]. The second is the spiritual advent, which happens daily into the minds of the righteous, since He is present constantly with the Church, hears her, helps and consoles her, concerning which Christ said, John 14[:18]: “I will not leave you orphans, but will come to you.” Again, [v. 23:] “If anyone loves Me, We will come to him and make Our dwelling with him.” The third advent of Christ is His glorious return to judgment, concerning which Isaiah 3[:14] says, “The Lord will come into judgment.” And Matthew 24[:30] says, “And they will see the Son of Man coming on the clouds of heaven, with much power and majesty.” It is useful always to consider these three advents of the Son of God—into flesh, the minds of the righteous, and for the last judgment—and to have them set forth in the Church for stirring up faith in minds, invocation, and the fear of God or repentance. St. Augustine says thus on Luke, “This time is called the Advent of the Lord for good reason: so that every believer will prepare himself and mend his ways, so that he may have strength worthily to celebrate the nativity of his God.”
Treasury of Daily Prayer, ed. Scot A. Kinnaman, (St. Louis: CPH, 2008) 1045.