Luke begins by referring to his former history of Christ now known as the Book of Luke and a short reminder of its contents.
Luke and Acts were written as historical letters to Theophilus a man of Greece [judging by the Greek name]. Theophilus must have shared these two letters with others and they began to circulate in the Ekklesia being made part of the cannon of scripture later at Pella.
Acts 1:1 The former treatise [the book of Luke] have I made, O Theophilus, of all that Jesus began both to do and teach, 1:2 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: 1:3 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: 1:4 And, being assembled together with them, commanded them that they [All the disciples not just the 10+1 [Judas being dead and Thomas not being present on that Sunday] who had already received the Holy Spirit on the Wave Offering Sunday or soon after.] should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me. 1:5 For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence.
Luke then bears record that they are commanded to preach Christ to all nations and that he is helping to do that by writing these historical letters. In doing this he passes on the commandment of Christ to spread the word to all nations and Theophilus would have taken this cue, to spread these letters from Luke.
The principle point of the true Gospel is the message of warning, repentance and SALVATION through Jesus Christ.
1:6 When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel? 1:7 And he said unto them, It is not for you to know the times or the seasons [In the first century], which the Father hath put in his own power. 1:8 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.
This was the last command of Christ as he ascended into heaven to be with God the Father until the time of the restitution of all things at the resurrection to spirit of the chosen.
1:9 And when he had spoken these things, while they beheld, he was taken up; and a cloud received him out of their sight. 1:10 And while they looked steadfastly toward heaven as he went up, behold, two men stood by them in white apparel; 1:11 Which also said, Ye men of Galilee, why stand ye gazing up into heaven? this same Jesus, which is taken up from you into heaven, shall so come in like manner as ye have seen him go into heaven.
The ten apostles [and later Thomas] had already received the Holy Spirit from Christ at the end of Wave Offering Sunday, however they were to wait at Jerusalem until all the disciples had God’s Spirit poured out on them and a public demonstration was made of the Spirit of God being poured out on some people, as an example of what would come after the day of the Lord (Joel 2:28).
The indication here is that Christ ascended into heaven from the Mount of Olives; the very place where he will soon descend to.
1:12 Then returned they unto Jerusalem from the mount called Olivet, which is from Jerusalem a sabbath day’s journey.
The eleven [Thomas now present] living together in Jerusalem, discussing the teachings of Christ and the scriptures, prepared themselves for the momentous events of Pentecost.
1:13 And when they were come in, they went up into an upper room, where abode both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James the son of Alphaeus, and Simon Zelotes, and Judas the brother of James. 1:14 These all continued with one accord in prayer and supplication, with the [several] women, and Mary the mother of Jesus, and with his brethren.
Peter then brings up the subject of replacing Judas; for it is written that each original apostle shall rule over a tribe of Israel in the Kingdom.
1:15 And in those days Peter stood up in the midst of the disciples, and said, (the number of names together were about an hundred and twenty,) 1:16 Men and brethren, this scripture must needs have been fulfilled, which the Holy Ghost by the mouth of David spake before concerning Judas, which was guide to them that took Jesus. 1:17 For he was numbered with us, and had obtained part of this ministry.
The end of Judas explained
1:18 Now this man purchased a field with the reward of iniquity; [He hanged himself and when cut down] and falling headlong, he burst asunder in the midst, and all his bowels gushed out. 1:19 And it was known unto all the dwellers at Jerusalem; insomuch as that field is called in their proper tongue, Aceldama, that is to say, The field of blood. 1:20 For it is written in the book of Psalms, Let his habitation be desolate, and let no man dwell therein: and his bishoprick let another take.
The others agreed that Peter was consistent with the scriptures and undertook to choose a twelfth apostle.
1:21 Wherefore of these men which have companied with us all the time that the Lord Jesus went in and out among us, 1:22 Beginning from the baptism of John, unto that same day that he was taken up from us, must one be ordained to be a witness with us of his resurrection.
They then chose two disciples with had been with them since John Baptist, by the criteria of their loyalty to Christ and their godliness, and who had been with Jesus from his baptism by John.
1:23 And they appointed two, Joseph called Barsabas, who was surnamed Justus, and Matthias.
Then the twelfth apostle was chosen by lot from among the qualified. Almighty God chooses his own apostles! Men falsely proclaiming themselves apostles without the fruits of an apostle; does not make them an apostle.
1:24 And they prayed, and said, Thou, Lord, which knowest the hearts of all men, shew whether of these two thou hast chosen, 1:25 That he may take part of this ministry and apostleship, from which Judas by transgression fell, that he might go to his own place. 1:26 And they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles.