Jesus died at Passover time, rising on the third day and appearing to his disciples over a period of 40 days before ascending into heaven. He had told them to wait in Jerusalem for ” the promise of the Father” (Acts 1:4), which he had previously explained to them. He was talking about the arrival of the Holy Spirit. At the Last supper he had talked at length about this Comforter/Counselor who would come to them once he was gone ( John 15—-16). Now it was about to happen.
When the Spirit arrived, there were three manifestations. First there was a sound like a violent wind. Then “tongues, as of fire” (verse 3) descended from heaven upon the heads of disciples. And then began to speak in other languages, which were understood by people in the international crowd visiting Jerusalem at the time.
There is some disagreement as to whether they were speaking specific languages or some heavenly language that was miraculously translated by the Spirit into the languages of multiple nations. In any case, listeners were amazed that they understood the message in their own language.
Of course, there are always scoffers. Some onlookers thought the disciples had to be drunk. So Peter began the most important sermon in Church history by saying, “Indeed, these are not drunk, as you suppose, for it is only nine o’clock in the morning” (verse 15).
Throughout scripture God often signals his presence with a fiery cloud. Even at Jesus baptism, the Spirit descended like a dove–that is, a vivid, flying brightness.