The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach, Until 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: To whom also he shewed 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: And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me. For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence. When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel? And he said unto them, It is not for you to know the times or the seasons, which the Father hath put in his own power. But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.
After the most gruesome humiliation and unthinkable suffering, the Savior rose triumphant over sin, hell, the devil, and death. Now He had to grant to His disciples infallible proofs that yes, indeed, He lives and all Scripture is true, authentic, and ratified in His Covenant Blood. Also knowing the enemy, both devils and humans, and the frailty of the human mind, the reality of His resurrection has to be perpetually engraved in their consciousness that no sand of time, lies, persecutions, delusions, and deceptions can dampen it. Not just His appearances, but His ascension and the descent of the Almighty Holy Spirit.