"Then He said to them, 'These are the words which I spoke to you while I was still with you, that all things must be fulfilled which were written in the Law of Moses and the Prophets and the Psalms concerning Me.' And He opened their understanding, that they might comprehend the Scriptures. Then He said to them, 'Thus it is written, and thus it was necessary for the Christ to suffer and to rise from the dead the third day, and that repentance and remission of sins should be preached in His name to all nations, beginning at Jerusalem. And you are witnesses of these things. Behold, I send the Promise of My Father upon you; but tarry in the city of Jerusalem until you are endued with power from on high.'"
The recent events that had transpired were exactly as Jesus had foretold them (and exactly as the Old Testament writers had recorded prophetically). The phrase, "while I was still with you," has reference to the time prior to His death when He was with them in the flesh constantly. His physical presence with them now was the exception and not the rule.
"And He opened their understanding, that they might comprehend the Scriptures" (Luke 24:45). Although some believe this illumination was of a miraculous nature, the next verse seems to suggest that He did it via discourse--just as He had already done with the two on the way to Emmaus (cf. 24:27).
Why was it "necessary for the Christ to suffer and to rise from the dead the third day"? There are at least three reasons: (1) It was God's will, (2) It had been prophetically foretold (and God cannot lie), and (3) It was necessary for the redemption of man. Praise be to God for His plan of salvation that He had in mind from the foundation of the world (cf. Rev. 13:8)!
Because of the work that Jesus fulfilled, "repentance and remission of sins [could and] should be preached in His name to all nations" (Luke 24:47). This great evangelistic work would begin in Jerusalem on the Day of Pentecost in Acts 2.
The apostles were witnesses of the matters that Jesus spoke. No one on Earth today can be a witness as they were since none today have seen the risen Christ. However, though we are not literal witnesses of the gospel message, we still have the responsibility of sharing it with the world.
"Behold, I send the promise of My Father upon you; but tarry in the city of Jerusalem until you are endured with power from on high" (Luke 24:49). Jesus here refers to the promise of the Holy Spirit (cf. John 14-16). They would be empowered in a special way in Jerusalem on Pentecost when Jesus baptized them with the Holy Spirit (cf. Mark 9:1; Acts 1:4,5,8; 2:4).
We will continue studying this narrative in our next lesson.