The Book of Acts is a declaration of the Word of God which was made known to Luke regarding practices (actions), and this account is not limited only to the twelve who had been chosen by the Lord Jesus Christ during his earthly ministry (Luke was not one of these twelve). It was revealed to Luke after the resurrection of the Lord Jesus Christ whom God raised from among the dead-people, and it includes details regarding the ascension of the resurrected Christ Jesus into heaven, the shedding-forth of the gift of holy spirit on the day of Pentecost, and includes how God’s children lived up-until the time of the Apostle Paul’s heralding-forth of the word pertaining to the kingdom of God from his own hired-house.
Acts 1:1 and 2:
1:1Indeed the first word I made concerning all-things, oh Theophilus, which Jesus began both to do and to teach .2continuously-until which day he was taken-up, having commanded the apostles by means of holy spirit whom he selected-for-himself,
Luke begins by referring back to the book which he had previously written by revelation from God which we sometimes call the “Gospel of Luke.” He also clearly states to whom he is addressing this book: to “Theophilus,” which is a name meaning “God’s loved” or “beloved of God,” and the word “loved” here means the friendly or brotherly kind of love (refer to Luke 1:3).
Indeed, Luke had previously written-down an account about all of the things which Jesus began (started) both to do (perform) and to teach (to instruct) when he was on earth carrying-out his ministry during that time, up to the day when Jesus was taken (received) upwards by God into heaven. Jesus’ ascension happened after Jesus had commanded (charged, enjoined) the apostles concerning certain things which were made known to him by the revelation of information from God through the holy spirit-life which he received when God raised him up giving him his new spiritual body. Jesus had previously selected (chosen, picked out) these twelve men for himself. These men were “apostles” because Jesus sent them forth on specific missions or assignments from himself towards other people.
Let us read two records from the Gospel of Luke: 1) regarding Jesus’ selection of his twelve, and 2) regarding his being taken-up into heaven.
And it came-to-pass in those days, that he went out into a mountain to pray, and continued all night in prayer to God.
And when it was day, he called unto him his disciples: and of them he chose twelve, whom also he named apostles;
Simon, (whom he also named Peter,) and Andrew his brother, James and John, Philip and Bartholomew,
Matthew and Thomas, James the son of Alphaeus, and Simon called Zelotes,
And Judas the brother of James, and Judas Iscariot, which also was the traitor.
And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high.
And he led them out as far as to Bethany, and he lifted up his hands, and blessed them.
And it came-to-pass, while he blessed them, he was parted from them, and carried up into heaven.
Returning to the Book of Acts, we read a more detailed account of what the resurrected Christ Jesus said to them before his ascension.
.3to whom also he presented himself living after his suffering in many sure-signs by means of forty days being seen by them and saying the-things concerning the kingdom of God,
It was to these twelve that the resurrected Christ Jesus also caused himself to stand beside or near-by, not as a dead-person but as living (actively being alive, having life) after his suffering. The word “suffering” refers to his experiences, what he was affected by – which included his arrest, being evilly and maliciously treated, his crucifixion, death, and burial.
The resurrected Christ Jesus, having received his new spiritual body, proved in a lot of sure-signs (within the sphere of action of many indubitable evidences, fixed or sure marks being the criteria or standards by which to ascertain the truth) that he was living through/during the period of forty days between his resurrection and his ascension into heaven. The number 40 is significant in-that from God’s point of view it is the period of probation and trial in order to prove a thing as being true.
He did this while causing himself to be seen by the apostles (physically seen with their eyes) and speaking to them the things concerning the kingdom of God (about all that is contained within God’s sovereignty, His kingly or royal dominion and government and rule and territory and power, etc, which of course is holy and spiritual because God is the Holy Spirit).
[Reference: Matthew 26:47-28:20; Acts 13:28-39, and 28:31.]