“I believe that Jesus Christ, true God, begotten of the Father from eternity, and also true man, born of the Virgin Mary, is my Lord…” (SC, The Second Article: Redemption).
What does it mean to have a Lord? To have a Lord means to owe all honour and obeisance to that person. Thus, when we confess Jesus is our Lord, we honour and obey Him above all things, just as we confess in the 1st Commandment, “We should fear, love, and trust in God above all things.” What does it mean to have Jesus as your Lord? As Luther writes, “It means that He has redeemed and released me from sin, from the devil, from death, and from all misfortune. Before this I had no lord or king, but was captive under the power of the devil” (LC II, 27). Thus, when Jesus came, He rescued you and me from the kingdom of the devil, “who called you out of darkness into His marvellous light” (1 Peter 2:9).
As we confess this about the second Person of the Holy Trinity, we also confess He is the only-begotten Son of the Father. It is a mystery we confess by faith. To reiterate, a secret is a thing hidden and therefore unknown; a mystery is a thing revealed (and therefore known) yet unfathomable. By faith, God has revealed the mystery of His Being in the Holy Trinity. We do not claim to fathom how Jesus is the only-begotten Son of the Father, but merely confess how He Himself has revealed Himself (e.g., Matthew 3:17; John 1:1, 9, 14, 18; 3:16; 5:18, 26; 10:30; 14:9; Hebrews 1:5; Colossians 1:17; Psalm 27:1; etc.). Jesus Christ, who is God the Son, is our Lord who reigns for all eternity. As our Lord, or King, He entered the battlegrounds of sin, death, and the devil—and although He died, He rose from the dead in victory to give us this same victory just like His (Romans 6:3-5).