The one hundred twenty disciples in the Upper Room had, of course, been much like any other group of believers in history. They had not all been equally spiritual or devoted, or faithful. Some had been more so than others, and where some had excelled in one virtue, others had excelled in another. Yet now they were all FILLED with the Spirit, from the least to the greatest of them.
The thoughtful student of Scripture will, of course, ask why all these believers were now filled with the Holy Spirit. Was it, perhaps, because they, as a group, had been more godly than those before them? The gospel records prove that this is not so. Peter boasted, Thomas doubted, James and John sought personal gain, and when our Lord was taken prisoner, “they all forsook Him and fled.”
Was it then because they had prayed long enough or earnestly enough for the Spirit to come upon them and take control? No; they had been instructed to go to Jerusalem, not to pray for the Holy Spirit to come, as some suppose, but to “wait for the [fulfillment of the] promise” regarding the Spirit (Acts 1:4,5) — and right here is the answer to our question.
The believers at Pentecost were filled with the Holy Spirit, not because they had prayed long or earnestly enough for the Spirit to come, but because the time had arrived for the fulfillment of the divine promise. The Old Testament prophets and the Lord Jesus had promised that the Holy Spirit should some day come to take control of God’s people (Ezek. 36:26,27), and that day had come. They were filled with the Spirit because God, according to His promise, had baptized them with the Spirit (Acts 1:5). source