Jesus had promised ((Luke 24:49)) his disciples that “the holy spirit” would “come upon” them ((Acts 1:8)) in a few days ((Acts 1:5)). They waited expectantly, remembering the Bible stories about God’s spirit filling great men like David ((1 Samuel 16:13)), Samson ((Judges 13:25, 14:6, 14:19, 15:14)), Gideon ((Judges 6:34)), Joshua ((Numbers 27:18)), and many other heroes ((Genesis 41:38, Judges 3:10, 11:29, 1 Samuel 10:10, 1 Chronicles 12:18, 2 Chronicles 24:20, Ezekiel 11:5)) and prophets ((Ezekiel 11:5, Daniel 4:8, Micah 3:8)). It had given them wisdom and power, and brought deliverance numerous times.
Prophecy said that the Messiah would have the spirit of God on him ((Isaiah 1:2)), and they had witnessed it themselves. When Jesus said the same spirit would be given to them, they could not help but recognize it as the fulfillment of an ancient prophecy ((Ezekiel 39:29, Joel 3:15)): God would restore Israel and pour his spirit on all, marking the dawning of a new age.
Yet even though scripture spoke often of God’s spirit, it was still a mystery to them. Its appearance had grown uncommon in the centuries since the last prophets. Those in power at the temple were certainly devout, but God’s spirit was not part of the routines of temple life. Those ministering at the altars carried out the traditions of a millennium, confident that they were following the will of God. Without the insights of the holy spirit, they could not conceive what God was preparing in an upper room at the edge of the courtyard.