To believe in Jesus is to accept the fact that He is our Savior. He is Son of Man (truly human) and Son of God (truly God). Faith is not blind guessing about what ‘might’ be so. Jesus assures Nicodemus,
“Truly, truly, I say to you, we speak of what we know, and bear witness to what we have seen, but you do not receive our testimony” (John 3:11).
Nicodemus hears the “testimony,” the true report of eyewitnesses. He sees solid evidence. By these He comes to believe.
The crowning evidence is Jesus’ resurrection. The dead body, which Nicodemus helped to lay in the grave, comes to life again. The risen Christ presents Himself to chosen witnesses: Peter, John, Paul and others, including 500 at one time (John 20; 1 Corinthians 15).
He presented himself alive to them after his suffering by many proofs, appearing to them during forty days and speaking about the kingdom of God (Acts 1:3).
“You will be My witnesses,” Jesus tells His apostles. The Holy Spirit makes sure that their testimony is accurate in every way (Acts 1; John 14,16; 1 Peter 1).
You and I have not seen Jesus in person. Yet our faith can be as strong as that of eyewitnesses. When Peter writes to readers like you and me, he describes them as enjoying faith equal to His own (1 Peter 1; 2 Peter 1). John assures us his writings can bring us to saving faith (John 20; 1 John 5). Hebrews 11:1 describes faith as “the conviction of things not seen.” We are sure – we believe – because the word of God is so completely dependable.
So faith comes from hearing, and hearing through the word of Christ (Romans 10:17).