(This post is an entry for the Catholic Carnival #121.)
Compare the story of the Tower of Babel (Genesis 11:1-9) with the first Christian Pentecost (Acts 2:1-11).
Why did God confuse the tongues of men at Babel? First, let us understand what the men were doing. We hear from the men themselves: "Come, let us build ourselves a city, and a tower with its top in the heavens, and let us make a name for ourselves, lest we be scattered abroad upon the face of the whole earth." (Genesis 11:4). I can spot two problems with their plan. First of all, it was God's will that mankind be "scattered abroad upon the face of the whole earth" (Genesis 11:4): He had already told us to "be fruitful and multiply, and fill the earth and subdue it" (Genesis 1:28)... three times (cf. Genesis 9:1, 7). Why, then, was man afraid of being thus scattered? Second, these men sought to make a name for themselves rather than for God. The city and tower were not for the glory of God, but for the glory of Man, and unless the LORD builds the house, those who build it labor in vain. (Psalm 127:1)
So God, seeing that mankind put His will beneath their own and sought to elevate their own names rather than the name of the One True God, confused their speech and scattered them abroad. This was a punishment of sorts for not keeping the covenant of filling the whole earth, and for preferring their own glorification over God's.
But on the day of Pentecost, after Jesus the Christ had been crucified, raised, and had ascended into Heaven, the Holy Spirit filled the Apostles and those with them (including Mary, the mother of Jesus, cf. Acts 1:12-14) and "confused their speech". God granted the same gift (not punishment) to these holy men and women again, so that His will might be fulfilled.
Jesus told his friends to "make disciples of all nations" (cf. Matthew 28:19). But how could they do so without being able to preach to those nations, using words the nations could understand? Here is the beautiful Wisdom of God displayed: yet another foreshadowing of the New Covenant in the Old. Just as the sacrifice of Isaac prefigured the sacrifice of Jesus, just as the ark of Noah prefigured baptism, just as the manna in the desert prefigured the True Bread from Heaven, so to did the confusing of tongues in Babel prefigure the confusing -- and understanding -- of tongues in Jerusalem.
The disciples of Jesus spoke in various tongues -- not the languages they already knew, but the languages of the devout Jews visiting Jerusalem, the men who were "Parthians and Medes and Elamites and residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the parts of Libya belonging to Cyrene, and visitors from Rome, both Jews and proselytes, Cretans and Arabians" (Acts 2:9-11). This was for the furthering of the Kingdom and the glorification of God, that all the world might know the saving power of Jesus Christ and believe in him.
Instead of building a city of men, now we are building the city of God.