This argument will be presented cumulatively.
The Promise is the Holy Spirit
you will receive power when the Holy Spirit comes on you can be read to mean:
- The Father is sending you the Holy Spirit
- One of the effects of receiving the Holy Spirit is you are given power (other effects include being led to truth, remembering what Jesus has taught, and so on, as described in the passion narrative of John)
This does not even require that the power one person receives from the Holy Spirit is identical to the power another person receives; this is a generic statement: the Holy Spirit brings power.
In this sense, then, Jesus is explaining what results from receiving the Holy Spirit. What is being promised is the Holy Spirit.
11, not 11 squared
The immediate audience of this statement is the apostles. The antecedent of the 3rd person plural pronouns & verbs throughout verses 3-12 can be found in verse 2:
unto the apostles whom he had chosen
That Mary, other women, and Jesus' brothers were also present is not impossible, but it is unlikely; the text does not require that they were there until the upper room scene in verses 13-14. However, the group of 120 is not identified at all until the subsequent pericope, beginning in verse 15. They do not appear to be the focus of any of the prior verses.
Since Judas Iscariot is dead, there appear to be 11 members of the audience, not ~120 (I know 11 squared is 121, not 120, but Luke did say about 120 =))
Conclusion 1: although the immediate audience is the apostles, the truth stated here is generally applicable: the Holy Ghost brings power.
We can then restate the question as: to whom is the Holy Spirit promised?
The Laying on of Hands
The book of Acts teaches that the Holy Ghost is given by the laying on of hands (e.g. Acts 8:17-19), and that not just anyone has the authority to do this (see Acts 8:20). If the ability to give the Holy Ghost by the laying on of hands could come through studying in a university, then Simon Magus' question actually looks pretty legitimate--he expects that his material goods can acquire this authority (like material goods can acquire a degree?). Simon Peter's condemnation, then, clearly indicates that this is an authority that worldly goods cannot acquire.
Who, in the present day, has the authority to give the Holy Ghost through the laying on of hands, is probably a question better suited for Christianity Stack Exchange.
Independent of this question, it is clear from the New Testament that numerous Christians, not just the apostles, received blessings through the Holy Ghost.
The Holy Ghost is not all or nothing
Multiple examples in the New Testament show that the blessings of the Holy Ghost are not all or nothing. Clearly something incremental was granted at Pentecost, yet the Holy Ghost was active prior to that time (e.g. Matt. 3:16, Matt. 16:17, Luke 24:31-32)
This incremental gift from God, often referred to as the baptism of the Holy Ghost is more immersive and came later, but the influence of the Holy Ghost was already present before Pentecost.
The New Testament does not suggest any prerequisite such as baptism or the laying on of hands, in order to be influenced by the Holy Ghost.
Conclusion 2: God gives power & blessings by degrees.
The Holy Spirit can presumably bless and give power to anyone who is open to it, but the baptism of the Holy Spirit--and the degree of power that comes from it--is a blessing that follows a clear order of operations.
Power vs. Authority
There may be value in offering a distinction between "power" & "authority".
- A person with a driver's license and a car has both the power & the authority to drive a car
- A person with a license but no car has the authority, but lacks the power to drive a car
- A person with a car but no license has the power, but lacks the authority to drive a car
I see no basis in this text (or other texts) that one must have the authority to bestow the Holy Ghost in order to be given any power at all from the Holy Ghost. Indeed, Paul evinces a belief quite to the contrary. E.g.
But the manifestation of the Spirit is given to every man to profit withal (1 Cor 12:7)
28 And God hath set some in the church, first apostles, secondarily
prophets, thirdly teachers, after that miracles, then gifts of
healings, helps, governments, diversities of tongues.
29 Are all apostles? are all prophets? are all teachers? are all
workers of miracles?
30 Have all the gifts of healing? do all speak with tongues? do all
interpret? (1 Cor. 12:28-30)
If gifts of the Spirit are given to every man, but not every man is an apostle (or prophet, or teacher, etc.), then clearly one can be given power through the Spirit without being given all powers (and without holding a certain office).
Indeed, the New Testament provides multiple examples of people who clearly had power and authority, but did not have apostolic authority--there were things they couldn't do (e.g. John the Baptist in Matt. 3:11, Philip in Acts 8:12-15).
Is the promise of "power" in Acts 1:8 only for the apostles or for Christians in all ages?
Blessings & power from the Holy Ghost are available for all people in all ages. However, this does not mean all blessings & power given to the apostles are given to all people in all ages.
The baptism of the Holy Spirit requires authority; the specific authorities granted to the apostles (including the authority of the laying on of hands) were not promised to all people in all ages (as evidenced by righteous Christians in the New Testament who did not receive them).
Who held/holds this authority beyond the historical period covered by the text is not answered by this passage.
Appendix: The Ends of the Earth
One could look at the 1st century Christians and conclude that, under heavy persecution, they failed the in the great commission to take the Gospel to the ends of the earth. However, I don't see it that way.
I argue in the parable of the farmer (parable, interpretation) that the apostles were commissioned to sprinkle the world with faith in Christ (but not necessarily to relay everything Jesus had ever taught or to answer every conceivable question), and in that mission they succeeded.
Although many (most) of the apostles would die martyr's deaths, they did not fail. Why?
Because Matthew, Mark, Luke, John, Paul, James, Peter, and Jude (and whoever wrote Hebrews) left a record of their testimonies of Christ, and the Bible has taken their testimonies to the ends of the earth, sprinkling the world with faith in Christ, as they were commissioned to do.