The devotion is from the Lutheran Confessions, specifically the Solid Declaration of the Formula of Concord VIII.85
85] In the little book concerning the Last Words of David, which Dr. Luther wrote shortly before his death, he says as follows: According to the other, the temporal, human birth, also the eternal power of God has been given Him; however, in time, and not from eternity. For the humanity of Christ has not been from eternity, like the divinity; but, as we reckon and write, Jesus, the Son of Mary, is 1543 years old this year. But from the instant when divinity and humanity were united in one person, the man, the Son of Mary, is and is called almighty, eternal God, who has eternal might, and has created and sustains all things per communicationem idiomatum for the reason that He is one person with the divinity, and is also true God. Of this He speaks Matt. 11:27: "All things are delivered unto Me of My Father"; and Matt. 28:18: "All power is given unto Me in heaven and in earth." To which Me? To Me, Jesus of Nazareth, the Son of Mary, and born man. From eternity I have it of the Father, before I became man. But when I became man, I received it in time, according to humanity, and kept it concealed until My resurrection and ascension; when it was to be manifested and declared, as St. Paul says,Rom. 1:4: "He is declared and proved to be a Son of God with power." John 17:10 calls it "glorified." Vol. 5, Wittenb. Germ., fol. 545.