Summary: Contrasting the transformation following the descent of the Spirit with life before His presence among the saints.
“A dispute also arose among them, as to which of them was to be regarded as the greatest. And he said to them, ‘The kings of the Gentiles exercise lordship over them, and those in authority over them are called benefactors. But not so with you. Rather, let the greatest among you become as the youngest, and the leader as one who serves. For who is the greater, one who reclines at table or one who serves? Is it not the one who reclines at table? But I am among you as the one who serves…’” 
“When they heard this, they were cut to the heart, and said to Peter and the rest of the apostles, ‘Brothers, what shall we do?’ And Peter said to them, ‘Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself.’ And with many other words he bore witness and continued to exhort them, saying, ‘Save yourselves from this crooked generation.’ So those who received his word were baptized, and there were added that day about three thousand souls.
“And they devoted themselves to the apostles’ teaching and the fellowship, to the breaking of bread and the prayers. And awe came upon every soul, and many wonders and signs were being done through the apostles. And all who believed were together and had all things in common. And they were selling their possessions and belongings and distributing the proceeds to all, as any had need. And day by day, attending the temple together and breaking bread in their homes, they received their food with glad and generous hearts, praising God and having favor with all the people. And the Lord added to their number day by day those who were being saved.”
The transformation was incredible! In the Upper Room, on the eve of His passion, Jesus’ disciples were bickering, squabbling, jockeying for position. At Pentecost, mere weeks after this despicable scene, these same disciples demonstrate unimaginable power and supernatural harmony as the greatest revival in the storied history of the Faith breaks out!
We readily understand what happened in the Upper Room. The disciples were human; they were like us! They had their petty jealousies and they were fueled by raw ambition. Some of them had sharp memories honed to recall every wrong and each insult. Some of them were aggressive and exceptionally ambitious; they knew that if you don’t promote yourself in this world, no one else will. Most of them were jockeying for place and position in the kingdom just as would any decent Canadian. We can imagine the conversations that evening.
“I deserve the place of honour,” asserted one.
“You don’t deserve anything,” another retorted. “I remember how you behaved that time in Caesarea.” They had such long memories, recalling every slight—real and imagined.
The disagreement was so intense that Jesus had to intercede. “The road to greatness lies through serving,” He said. “If you want to be great, then live as though you are a servant.”
What took place in the Upper Room is so typical of mere mortals. Despite our union with Christ, we are yet infected with the virus of “self.”
It is much harder for us to understand what happened at the festival of Pentecost. All those people gathered from many points of the globe; and there was a spirit of solidarity among the disciples. There is no fussing or bickering, only a witness to Jesus and all that God had accomplished at the cross. Then the Spirit of God fell upon them so mightily that the faithful ever since have celebrated what happened as part of its annual calendar of remembrance.
They moved from quarrelling to concert, from contention to unity. What happened? What could account for such a change? Luke, who reported both incidents, makes the reason perfectly clear. Between the Upper Room and the festival of Pentecost, between the quarrelling and the unity, they had two unforgettable experiences—the crucifixion and resurrection of Jesus.
These disciples had stood by and watched the Master as He was seized and delivered over to the Jewish leaders. They had seen the One whom they had followed for years, the One with whom they had walked and worked and eaten and slept, the One who had been their Master and teacher as He had been nailed mercilessly to a rough Roman cross and raised against the Judean sky. They had seen Him suffer while nailed to that rough cross. They had watched His life drain out drop-by-drop as His blood stained the sand and the rocks at the foot of the cross.