(NASB) Acts 1:9 “And after He said these things…” 

  What were “these things” that Jesus said?  Using Acts 1:4-5, 7-8
1- wait in Jerusalem
2- wait for what the Father had promised
3- you will be baptized with the Holy Spirit, not many days from now
4- the time of restoring the kingdom of God to Israel is unknown by any but the Father
5- the Holy Spirit will come upon you
6- the apostles would be witnesses of Jesus Christ in Jerusalem, Judea and Samaria and to the remotest corners of the world
  After Jesus had said “…these things, He was lifted up while they were looking on, and a cloud received Him out of their sight.”  (NASB Acts 1:9) 

  Those apostles had been given proof of Jesus’ death, His burial and His resurrection…and now they had seen His ascension into heaven.  They were eye-witnesses to all of those events. 

  From this group of witnesses came most of the books of the New Testament.
The Gospels of Matthew, Mark (many believe that the book of Mark had Peter’s input) and John.  The letters of 1 & 2 Peter, 1, 2 & 3 John, the book of Revelation penned by John – these men were eye-witnesses of Jesus Christ.

  What about Paul?  He was met by Jesus on the road to Damascus.  He was called by Jesus to be His witness to the Gentiles, the Jews and to kings.  Paul wrote most of the letters in the New Testament.  (Acts 9:1-19)  Paul witnessed the resurrected, ascended Jesus in a way none of the other men experienced. 

  You might ask about Dr. Luke who wrote the books of St. Luke and the Acts.  Luke interviewed the people who had known the Lord Jesus. 
(NASB) Luke 1:1-4 “INASMUCH AS many have undertaken to compile an account of the things accomplished among us,
2 just as they were handed down to us by those who from the beginning were eyewitnesses and servants of the word,
3 it seemed fitting for me as well, having investigated everything carefully from the beginning, to write it out for you in consecutive order, most excellent Theophilus;
4 so that you may know the exact truth about the things you have been taught.” 

  His historical perspective came from the people intimately involved in the life of Jesus. 

  The followers of Christ are witnesses to the living Lord Jesus as Savior and Redeemer, Father and Counselor…hallelujah!  What a God we worship and serve!  What privilege to be witnesses to the truth of God’s words, because we are to be living them, day by day.