How to be certain of the gospel

Luke mentions proofs of Jesus' credentials, not only in his gospel but in the book of Acts.

"After his suffering, he showed himself to these men and gave many convincing proofs that he was alive. He appeared to them over a period of forty days and spoke about the kingdom of God."

Acts 1:3 And both Acts and the gospel of Luke were letters written to a man called "Theophilus". Why? Luke tell us why at the beginning of his gospel saying,

"Many have undertaken to draw up an account of the things that have been fulfilled among us, just as they were handed down to us by those who from the first were eyewitnesses and servants of the word. Therefore, since I myself have carefully investigated everything from the beginning, it seemed good also to me to write an orderly account for you, most excellent Theophilus, so that you may know the certainty of the things you have been taught." Luke 1:1-4

It was to impart a sense of certainty.

The reason why faith comes from hearing the message is because the message itself provides forensic evidence, evidence in the form of eyewitness testimony of miracles, evidence which gives a sure foundation for faith, evidence of validating Jesus' testimony, evidence constituting Jesus' credentials.

"Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the miracles themselves." John 14:11

"if you confess with your mouth, "Jesus is Lord," and believe in your heart that God raised him from the dead, you will be saved." John 10:9