Acts 1:3

To whom he also showed himself alive after his passion by many proofs, appearing unto them by the space of forty days. And speaking the things concerning the kingdom of God.

Many proofs ... It is regrettable that the KJV rendition of "many infallible proofs" was not followed here; for, while it is true that "infallible" is not in the Greek text, that meaning "is really included in the noun (proofs), which was used by Plato and Aristotle to denote the strongest proof of which a subject is susceptible."[5]

The space of forty days ... The teaching here is that at intervals throughout a period of forty days Jesus made frequent appearances to the apostles.

Unto the apostles ... Significantly, Jesus never appeared to any of his enemies. "The testimony of them that knew him best would be stronger than that of mere acquaintances."[6] Furthermore, the refusal of the Pharisees to believe, even after Lazarus' resurrection, proved that it would have done no good for Jesus to have appeared to the wicked and self-hardened priests. Jesus himself said, "Neither will they be persuaded, if one rise from the dead" (Luke 16:31).

"This implies, obviously, much unrecorded teaching."[7] Certain specifics, however, are clearly visible in what is recorded, such as: (1) that Jesus is the fulfillment of Old Testament prophecy (Luke 24:47,44,45); (2) that all men, including the Gentiles, were to be received into the kingdom through their faith and submission to baptism (Matthew 28:19,20 and Mark 16:15,16); (3) that Jesus would be with his church perpetually, watching over his followers providentially (Matthew 28:20 and Mark 16:17ff), etc.

Concerning the kingdom ... Not only here, but in Acts 8:12; 20:25; and Acts 28:31, Luke identified the gospel in this manner.

[5] John Peter Lange, Commentary on Acts (Grand Rapids, Michigan: Zondervan Publishing House, 1866), p. 8.

[6] R. E. Walker, Studies in Acts (College Press, Reprint Library, n.d.), p. 10.

[7] E. H. Plumptre, The Acts of the Apostles (Grand Rapids, Michigan: Zondervan Publishing House, 1959), p. 1.