From The Acts of the Apostles:
"When the day of Pentecost had come, they were all together in one place. And suddenly, a sound came from heaven like the rush of a mighty wind, and it filled all the house where they were sitting. And there appeared to them tongues as of fire, distributed and resting on each one of them. And they were all filled with the Holy Spirit and began to speak in other tongues, as the Spirit gave them utterance."
Acts of the Apostles 2:1-4 (RSV)
From a sermon by a 6th century author:
"The disciples spoke in the language of every nation. At Pentecost God chose this means to indicate the presence of the Holy Spirit: whoever had received the Spirit spoke in every kind of tongue. We must realize, dear brothers, that this is the same Holy Spirit by whom love is poured out in our hearts. It was love that was to bring the Church of God together all over the world. And as individual men who received the Holy Spirit in those days could speak in all kinds of tongues, so today the Church, united by the Holy Spirit, speaks in the language of every people.
Therefore if somebody should say to one of us, "You have received the Holy Spirit, why do you not speak in tongues?" his reply should be, "I do indeed speak in the tongues of all men, because I belong to the body of Christ, that is, the Church, and she speaks all languages. What else did the presence of the Holy Spirit indicate at Pentecost, except that God's Church was to speak in the language of every people?"
Prayer: Come Holy Spirit. Fill the hearts of your faithful and kindle in us the fire of your love. O God, who by the light of the Holy Spirit did instruct the hearts of the faithful, grant that by the same Holy Spirit we may be truly wise and ever enjoy his consolations, through Christ Our Lord. Amen.