Acts 2:1-13 is Luke's narration of the descent of the Holy Spirit upon the disciples. As the Holy Spirit descended upon Jesus before his ministry commenced so now at the beginning of the work of the Church, the Spirit descends on the disciples with power.
The day is Pentecost, a Jewish feast commemorating the fiftieth day after the Feast of the Unleavened Bread/Passover and which celebrated the giving of God's gift. For the Jews the gift was the Law given to Israel through the mediation of angels. For the first Christian community, the gift -- the one promised by the Father (cf. Luke 24:49) -- is the Holy Spirit.
One hundred twenty disciples were gathered in one place -- the "upper room", where they were staying (cf. Acts 1:13). Luke gives us an idea of who these people were:
Peter and John, James and Andrew, Philip and Thomas, Bartholomew and Matthew, James of Alpheus and Simon Zelotes and Jude the brother of James.
All these were persevering with one mind in prayer with the women, and Mary the mother of Jesus, and with his brothers.
The coming of the Holy Spirit is described in terms of a strong wind and fire the tongues of which radiated on each of the disciples. Dillon (JBC, p. 731) points out that the description of the events narrated by Luke is similar to the Septuagint version of Isaiah 66:15-20.
For behold the Lord will come with fire,
and his chariots are like a whirlwind,
to render his wrath in indignation, and his rebuke with flames of fire.For the Lord shall judge by fire,
and by his sword unto all flesh,
and the slain of the Lord shall be many.They that were sanctified,
thought themselves clean in the gardens behind the gate within,
they that did eat swine’s flesh, and the abomination, and the mouse:
they shall be consumed together, saith the Lord.But I know their works,
and their thoughts:
I come that I may gather them together
with all nations and tongues:
and they shall come and shall see my glory.And I will set a sign among them,
and I will send them that shall be saved,
to the Gentiles into the sea,
into Africa,
and Lydia them that draw the bow:
into Italy, and Greece, to the islands afar off,
to them that have not heard of me,
and have not seen my glory.
And they shall declare my glory to the Gentiles:And they shall bring all your brethren out of all nations
as a gift to the Lord,
upon horses, and in chariots, and in litters,
and on mules, and in coaches,
to my holy mountain Jerusalem -- says the Lord --
as when the children of Israel should bring an offering in a clean vessel
into the house of the Lord.
Seen in the light of this Isaianic passage, the descent of the Holy Spirit has the character of an eschatological judgment upon those who think themselves sanctified but at the same time, it is God's act of gathering in the nations. This two-fold characteristic of God's coming in fire will be echoed later by Peter in vv. 14-36 as he explains to the representatives of the nations (see vv. 9-11) that the noise they have heard is the sign of God's pouring out his Spirit in the last days and that all are invited to gather around the Messiah, Jesus whom God has made Lord and Christ. (In John's gospel, the Spirit's work -- among others -- is described in forensic terms as one who will accuse, vindicate and condemn. See John 16:7-11) The oracle speaks also of "a sign among them" that He will send out to the Gentiles, those who "have not yet seen my glory". This "sign" -- a group of people -- will declare His glory to the Gentiles and will also bring their dispersed brothers back to Jerusalem. Looking at this through the eyes of Luke, the "sign" is the group of disciples who were empowered to speak to the nations.
With the coming of the Holy Spirit, the disciples were enabled to speak in different tongues. This phenomenon can refer on the one hand to ecstatic speech in tongues (glossolalia) or, on the other, to intelligible speech in foreign tongues. But whatever it means one thing is certain: the descent of the Holy Spirit also brought with it the healing of the divisions created at Babel (see Genesis 11:1-9). On that first Christian Pentecost, the community of disciples becomes the place where the different languages of men are forged together for a united and uniting proclamation of the gospel that is Jesus Christ. With the power of tongues, the disciples are then able to proclaim the gospel of repentance and forgiveness of sins to all nations (see Luke 24:47).
On the day therefore when the dispersed sons of Israel came to Jerusalem to celebrate the giving of God's gift, they were met by the core group of the new Israel, the remnant foretold in the prophecies of old (Is. 10:20.21.22;11:11.16;37:32;46:3;Jer. 15:11;23:3;25:20;31:7;Mic.2:12;4:7;5:3.8;Zep. 2:9;3:13;Zec 8:12) through which God's people will be reconstituted. Paul later on will write about this reconstitution -- as he interprets Psalm 68:18 -- in terms of Christ and His Body, the Church (Eph. 4:4-13).
The Holy Spirit comes as a power from on high that propels the community of disciples towards the nations. The rest of the Book of Acts will illustrate how He (She? In Hebrew, "ruah" is feminine!) will move the evangelizers of those early times from Jerusalem, to Samaria, and to the ends of the earth. Indeed the "Acts of the Apostles" is not so much about the apostles as about the Holy Spirit who acts in human history as Gift of God, Promise of the Father, Companion and Counsellor to the apostles (see Acts 8:29.39;10:19 [=11:12];16:7;21:4).
