Luke was not speaking as an eyewitness but as a
reporter who compiled eyewitness accounts of Christ's life,
death and resurrection. The gospel of Luke is the first volume
of a set. Volume 2 is the Book of Acts, and likewise Luke starts
that volume in a similar manner, speaking as a reporter
compiling eyewitness accounts, and in fact writing to the same
person - Theophilus.
Acts 1:1-3 In my former book, Theophilus, I wrote about all that Jesus began to do and to teach until the day he was taken up to heaven, after giving instructions through the Holy Spirit to the apostles he had chosen. 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.
The Gospels and the book of Acts are a compilation
of affidavits of eyewitnesses. Luke himself was an eyewitness of
some events in the book of Acts, being one of Paul's traveling
companion. In Col 4:14 Paul refers to him as "the beloved physician". In Philemon
1:24 Paul refers to him as "my fellow laborer". And in 2Tim
4:11 When Paul suffered imprisonment and persecution near
the end of his life, Paul writes to Timothy saying, "Only Luke is with me."
These initial statement Luke makes in his gospel
and in Acts reveal something characteristic of Christianity
which should not be overlooked or trivialized. Namely that the
foundation of our faith is not propositional truth but historic
facts. The basis of belief in Christ is not because He says we
should believe on Him for eternal life, but rather because of
His miracles, His fulfillment of prophecy, and especially His
resurrection from the dead.
This is unique to Judeo-Christianity among the world's religions. For other religions, like Islam, merely have propositions, but no proof. And in contrast to other religions which rely upon blind faith, Christian faith is not blind faith. "God has set a day when He will judge the world with justice by the Man He has appointed. He has given proof of this to all men by raising Him from the dead." Acts 17:31 Thus above Luke speaks of "many convincing proofs". Likewise John writes in his gospel, "Jesus did many other signs in the presence of His disciples, which are not written in this book; but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in His name." John 20:30,31 In fact Jesus discouraged belief which had no proof. "Do not believe me unless I do what my Father does. But if I do it, even though you do not believe me, believe the miracles, that you may know and understand that the Father is in me, and I in the Father." John 10:37,38
Note here in Luke 1:4 Luke speaks of his objective
in compiling these accounts, "So that you
may know the certainty of the things you have been taught." Biblical
certainty is not based upon blind faith, but upon the veracity
of historical facts.