We teach that the Word of God which spoke all things into existence (Genesis 1:3), who has always been and always will be, assumed a human nature in the womb of the Virgin Mary (John 1:1-5 ,14-18). Therefore, in the second person of the Holy Trinity, there are two natures - the divine nature and the human nature - inseparably joined in one person. There is one Christ, true God and true man, who was born of the Virgin Mary, truly suffered, was crucified, died, and was buried. He did this to reconcile the Father to us and to be a sacrifice, not only for original sin (the words the church uses to teach that ever since the fall of Adam (Romans 5:12) humankind is born without the fear of God, without trust in God, and with the inclination to sin) but also for all the sin we commit in this life.
This same Jesus descended into hell (1 Peter 3:18-22 ) not as further punishment for sin but as the One victorious over sin, death and Satan, and physically rose again on the third day. Afterward, He ascended into heaven to sit at the right hand of the Father. There He forever reigns and has dominion over all creatures. This same Christ will openly reappear again to judge the living and the dead.
We teach that Jesus is God because, when we look at the Bible, we see Jesus doing things only God can do: things such as miracles, dying on the cross for the sins of the world, rising back to life after three days in a tomb, and ascending into heaven until He reappears on the Last Day. All these events point to an historical Jesus who did historically verifiable, yet incredible things. (We find historical evidence for His miracles, His death and His resurrection not only in Scripture, but in other documents as well, including documents opposed to Christ and His teachings. Those disagreeing with all this either during or right after Jesus’ death and resurrection could have refuted what was written about Jesus and put an end to “the Christian myth.” But they didn't, in part because what happened was verifiable. This points to one simple fact: Jesus is unique in history. (John 3:16 ; 1 John 4:9-10).
When we say that Jesus is the only begotten Son of the Father we use the word “only begotten” to explain the relationship of the Son to the Father. It is not intended to give the idea that there was a time when the second person of the Trinity was not. Think of it as eternally begotten without reference to time.
Hence we Lutherans confess that “Jesus Christ, true God, begotten of the Father from eternity, and also true man, born of the Virgin Mary, is my Lord, who has redeemed me, a lost and condemned creature, delivered me and freed me for all sins, from death, and from the power of the devil, not with silver and gold but with His holy and precious blood and with His innocent sufferings and death, in order that I may be His and live under Him in His kingdom, and serve Him in everlasting righteousness, innocence, and blessedness, even as He is risen from the dead and lives and reigns to all eternity.” (Luther’s Small Catechism; John 1:1 , 14; Acts 20:28 ; Hebrews 9:22 ; 10:10-17; 1 John 4:9-10 ).