Acts 2

So, not only was the attention of the disciples gained, but the crowd outside as well. Then "cloven tongues, like as of fire" descended on each of them and they began to speak in tongues.
             The Old Testament prophets often had their first mission preceded by signs to prove to the people that they were prophets. The law on Mt. Sinai was given in fire. Ezekiel's mission was confirmed by a vision of "burning coals of fire" (ch 1:1) and Isaiah's by a "coal of fire" touching his lips (ch 6:7). John the Baptist said, "He shall baptize you with the Holy Ghost and with fire." Cloven tongues represented the Holy Ghost and when it came down on them, they began to speak with other tongues as the Spirit spoke through them (Acts 2:1-4). This is the initial sign of receiving the Holy Ghost and, as with the prophets, the sign that God resided within them. Jesus himself says that tongues is a sign that will follow a believer of the Gospel in Mark16:17. The prophets Joel (Joel 2:28-29) and Isaiah (Isaiah28:11), inspired by God, had foretold the arrival of the Holy Ghost many years earlier. Peter refers to this fact when he references Joel's prophecy in verse 17. He knew the devout Jews gathered there would know of the prophecy, and he wanted there to be no question about what was happening. He wanted them to know assuredly that "this is that". This was not just some freak occurrence or a bunch of over-excited people, this was the Spirit of God being poured out upon humankind just as promised. The outpouring was also foretold by John the Baptist in Matthew 3:11.
             One reason why God chose other tongues as the initial sign of receiving the Holy Ghost is that speaking in tongues is an immediate, outward evidence. Peter and the six Jewish Christians who went with him to Ceasarea knew that the Gentiles had received the Holy Ghost, not because of their temperance, gentleness, love, or peace, but because they heard them speak with tongues and magnify God (Acts 10:46).