Summary: The church's birthday occurred on the first Pentecost. This sermon studies what occurred at that first event.
Other Ref.: The Bible Knowledge Commentary
a. Today we are celebrating a major event in the Christian calendar, for it is literally our birthday.
(1) It is the day we know as Pentecost.
(2) Its inception was simple, but powerful.
(3) It did not start as any birthday party we have come to know and love.
(a) The party goers did not go to a residence festive with gay decorations.
(b) There was no cake and ice cream.
(c) No party invitations were mailed.
(d) It started with a simple command, “Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about. For John baptized with water, but in a few days you will be baptized with the Holy Spirit.” (Acts 1:4 (quickview) )
b. History of Pentecost
(1) Old Testament
(a) Not a new concept, but rather a transformation of a feast celebrated in O.T. times.
(b) In the Hebrew O.T., the customary name is the Feast of Weeks. It was regarded as the second of three obligatory observances, coming between the Passover and the Feast of Tabernacles.
(d) Dating of this festival also suggests its original agricultural context. Deuteronomy 16:9 (quickview)  required that it be dated seven weeks “from the time you first put the sickle to the standing grain.”
(2) New Testament
(a) In the Hellenistic period, 300 B.C.-A.D. 300, the feast began to lose its association with agriculture and came increasingly associated with religious history of the Hebrew people. The book of Jubilees, continuing to refer to it as ‘first fruits,’ identifies it with the covenant between God and Noah.
(b) Probably after the destruction of the Temple in A.D. 70 Pentecost finally transformed into an observance of the giving of Torah on Mount Sinai. Exodus 19:1 (quickview)  was interpreted to mean that the interval between Passover and the arrival at Sinai was fifty days. Thus, in Judaism, Shavuot continues to be an observance of thanksgiving for Torah.
c. Today, Pentecost has a totally different meaning for Christians. It is associated with power and excitement. In fact, it is so significant a denomination has been founded entirely on it.
2. Unusual Party Preparations
a. As we look in on the preparations for the first celebration of this wonderful birthday party, we find a somber scene. As I said earlier:
(1) There were no festive decorations.
(2) There were no party hats and balloons
(3) There was no cake and ice cream.
b. What there was, was a room of frightened men.
(1) Verse 19—the disciples were together with the doors locked, afraid of the Jews. What a contrast to their boldness when Christ was among them or 7 weeks in the future after Pentecost.
(2) Verse 22—the image and wording of breathing on them recalls God’s creative work in making Adam. This reception of the Spirit was in anticipation of the day of Pentecost and should be understood as a partial, limited gift of knowledge, under standing, and empowerment until Pentecost, 50 days later.