God’s purpose for the gift of tongues was three-fold.
Sign of authority.
Being one of the miraculous sign gifts, it confirmed the authority of the Apostles. The Apostles were used to give new revelation to the newly formed church. Since the New Testament had not yet been written, signs and wonders accompanied their teaching to authenticate them and their message (Acts 2:42-43).
Sign of blessing.
The sign gift of tongues showed to the Jews that other groups were included in the “church”. Both the Samaritans (Acts 8:5,15,16) and the Gentiles (Acts 10:44-48) spoke with tongues when they received the Holy Spirit. Speaking in tongues in that day was such a rare event that when Peter was called upon to defend his ministry to the gentiles in Acts 11:1-18; he said, “And as I began to speak, the Holy Spirit fell on them as on us at the beginning.” Peter had to refer them back to Pentecost to explain the manner in which they received the Holy Spirit and spoke in other languages.
Sign of judgment.
This was the primary purpose of the sign gift of tongues. It was given to warn the nation of Israel of coming judgment (I Co. 14:21-22 with Is. 28:11-12). In Isaiah 28:11- 12 the foreign language of the Assyrians was to serve as a judgmental sign to the Israelites, a proof that God was chastening (see Deut. 28:49, Jer. 5:15). To be addressed in another language was a symbol of judgment in the Hebrew mind. It meant that they hadn’t listened when God spoke to them in Hebrew, so now He was going to speak to them in languages they did not understand. In Acts 2:36 Peter announced that Israel was under judgment for their rejection of the Messiah and the sign gifts of the Spirit were authenticating factors, confirming that he was right. In AD 70, Rome destroyed Jerusalem and dispersed the Jewish people.
When the purpose of the gift was fulfilled the gift gradually ceased. The gift of tongues as practiced in the New Testament is no longer normative.