Acts 1:1-3

1The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach, 2Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen: 3To whom also he showed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God:

Comment: Luke ties this book, Acts, to his gospel, Luke. The comments concerning Luke 1:1-4, wherein Luke claims to have thoroughly investigated everything including eyewitness accounts of Jesus resurrection, apply here as well. In the passage at hand Luke indicates a specific period of 40 days during which the resurrected Jesus visited his disciples. Having a specific period of time designated in the account gives it credibility as specifics are a significant means for supporting or denying assertions. Further, Luke goes on to state the resurrected Jesus spent a good portion of that time dispelling any doubts that he had been raised from the dead. Surely Jesus' continued appearances to dispel doubt would not have been needed had not the resurrection been something that stretched the willingness to believe in the disciples themselves. The need for assurance among the original disciples, in itself, provides credibility to the account. False accounts do not typically contain doubts about their veracity since one of the main ideas is to divert attention from scrutiny. Expressing doubt in the narrative highlights the need the disciples themselves had for believing Jesus was raised from the dead. And, more importantly, that need was satisfied.