To these He also (a)presented Himself alive after His suffering, by many convincing proofs, appearing to them over a period of (b)forty days and speaking of the things concerning the kingdom of God.
v.3 (a)Jesus showed Himself to those that believed in Him. This fulfilled the promise that He made to His disciples and anyone who would put their hope in Him,
"that He would manifest Himself to those that believe in Him", Jn 14:21.
Peter confirms this and gives us a better insight to these forty days when he explains to Cornelius that Jesus was raised from the dead on the third day and was granted by the Father that He become visible; not to all people, but only to those that believe on Him, Acts 10:40-41. I find this fascinating because this explains why people did not recognize Him after the resurrection. He had the ability to only reveal Himself to those whom He wished to be revealed, to those who believed in Him. It is quite possible, and I believe it to be a fact, that no one saw the resurrected Jesus if they did not believe. Thomas may be the one exception to that, but still Thomas believed Jesus to be the Messiah, he just could not believe that He had risen from the dead unless he was to see it for himself. The story of Thomas is more than just a man that didn’t believe Jesus was raised from the dead; it was a foreshadowing to the end of this age when there will be us, the Church, who believed in Jesus without seeing Him, and there will be others who only believe because they saw Him coming out of the heavens and will be reigning as King over His kingdom here on the earth. Those people who only believed by seeing will still spend eternity with Jesus. But even better are those who believed without seeing, these are they that will be blessed, Jn 20:29.
The following is a list in order of the appearances of Jesus after the resurrection:
v.3 (b) Forty days is an interesting number. We see this appear all over our bible; specifically in some sort of a preparatory state. For example: