Why do you believe in Jesus, the Son of God?
Answer: First, seeing as the Son is of one substance and like glory with the Father, I rightly place my trust in Him. As the Scriptures testify, “I and the Father are one” (John 10:30), and “You believe in God, believe also in me” (John 14:1). See also Romans 9:5 and 1 Timothy 1:16, 17.
3 For I could wish that I myself were accursed, separated from Christ for the sake of my brethren, my kinsmen according to the flesh, 4 who are Israelites, to whom belongs the adoption as sons, and the glory and the covenants and the giving of the Law and the temple service and the promises, 5 whose are the fathers, and from whom is the Christ according to the flesh, who is over all, God blessed forever. Amen. Romans 9:3-5
16 Yet for this reason I found mercy, so that in me as the foremost, Jesus Christ might demonstrate His perfect patience as an example for those who would believe in Him for eternal life. 17 Now to the King eternal, immortal, invisible, the only God, be honor and glory forever and ever. Amen. 1 Timothy 1:16, 17
Second, I believe this because it is the Father’s command that we believe in the Son. We are firmly to trust Him that through Him and for His sake this salvation comes to us and we are received into grace, heard, and saved. For the command of the Father from heaven, “This is My beloved Son, in whom my soul is well pleased” (Matthew 3:17), carries with it the promise that through His Son His heart is pleased with us. We hear the Son explain the command and promise of the Father as follows: “This is the will of the Father who sent me, that everyone who sees the Son and believes in Him has everlasting life; and I will raise him up at the last day” (John 6:40). The Father is so serious about this command that He attaches eternal punishment to it for those who do not obey it: “He who does not believe the Son shall not see life, but the wrath of God abides on him” (John 3:36). Likewise John 8:24 “If you will not believe that I am He, you will die in your sins.” See also 1 John 2:28.
28 Now, little children, abide in Him, so that when He appears, we may have confidence and not shrink away from Him in shame at His coming. 1 John 2:28
Thus I confess that I believe in the Son of God, that from the heart I submit to His command and gracious promise of the Father, and that I desire, miserable and unworthy though I be, to be accepted for the sake of this eternal Son. I also confess that I do not wish to add to my manifold sins this greatest sin of all — the rejection of the Son of God. Rather, I heartily desire to withstand all unbelief and submit to the command of the Father to listen to the Son and trust Him. In this Son He will be pleased with me, as St. Paul says, “In Him you are complete” (Colossians 2:10).
Caspar Olevianus, A Firm Foundation, p.34