Summary: The significance of the Pentecost. We need the rely upon the Holy Spirit for church growth. Purpose of church is to communicate God’s love to a lost world.
Jesus died and rose again on Easter Sunday. He spent 40 days on earth with His disciples. Acts 1 (quickview)  recorded His last words to His followers. And He was taken up before their very eyes - that was His ascension. Jesus specifically asked them to "wait for a gift my Father promised" (1:5) and this gift is the Holy Spirit.
Ten days later - the day of Pentecost, they received the gift. Today is Pentecost day - 50th day after Easter. We remember the gift - the Holy Spirit - that God poured out upon the Church. This is important because Pentecost is the birthday of the Church. Jesus asked His followers to wait for this day to come before they even do anything.
Pentecost was a once-in-history event, and we can learn at least TWO things from this event.
(1) THE CHURCH WAS ESTABLISHED BY GOD
It was an act of God. Not an institution of man.
Simon Peter did not called a meeting of the apostles & said to them, "Now, we are here today to talk about whether or not we ought to start a Church." They did not discuss it. Nor were they prepared for it. No one would actually dare to proposed it, especially in Jerusalem. Where the religious leaders were out to destroy those who preach about Jesus.
So what really happened that day? The Word said, "Suddenly, a sound like the blowing of a violent wind came from..." (2:2). Where? "From heaven." It was an act of God. He started the Church! God brought the Church into being, and it is His Church.
And we saw that right after the outpouring of the Holy Spirit, the first person to speak out - the first spokesman of the Gospel of Jesus Christ - was Simon Peter (2:14ff.).
"But what about you?" he asked. "Who do you say I am?" Simon Peter answered, "You are the Christ, the Son of the living God."
Jesus replied, "Blessed are you, Simon son of Jonah, for this was not revealed to you by man, but by my Father in heaven. 18And I tell you that you are Peter, and on this rock I will build my church, and the gates of Hades will not overcome it."
At Pentecost the Holy Spirit came as He had never come before. In the past, He came upon individuals, inspiring prophets and priests. Now He was given to all who place their trust in Jesus, allowing us all to share God’s message to the world - that "everyone who calls on the name of the Lord will be saved." (2:21)
The signs were not new - the Wind and the Fire and the Tongues - they had all been associated with God’s Spirit in the Scriptures, but here the Spirit manifested Himself in greater intensity, with greater power, and greater scope - He comes upon the whole community of believers. There was a sense of overflowing fullness. He now dwells in every believer.
This was God’s gift to the Church. WHY? Jesus said it - 1:8 "you will receive power when the Holy Spirit comes on you and you will be my witnesses..."
Bro & Sis, it is very obvious here - we cannot grow the church without Him. It is not an institution created by man. Man did not establish this organisation called the church. God did. In fact, it is more an organism rather than an organisation. It grows because it has life - the new life in Christ, born-again life. Today, we are all called the church because our lives have been transformed by the power of the Holy Spirit.