Fifty (50) days from the Sabbath after Passover, is Shavuot or the Day of Pentecost. Rabbinic scholars believe that it was on this day that God visited His people after their exodus from Egypt and, through Moses, brought the Law down from Mount Sinai. This earthshaking day of visitation, trembling, and betrothal is the birthday of the nation of Israel. Moses brought down the Torah or Law for the nation. But, because of their rebellion 3,000 died under the Law that day.
Acts 2 describes a gathering of Israel for Shavuot …They have come from far and wide….from every nation under heaven. But this crowd is bewildered because each one hears their own language being spoken. 7 Utterly amazed, they asked: “Aren’t all these who are speaking Galileans? 8 Then how is it that each of us hears them in our native language? 9 Parthians, Medes and Elamites; residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia,[b] 10 Phrygia and Pamphylia, Egypt and the parts of Libya near Cyrene; visitors from Rome 11 (both Jews and converts to Judaism); Cretans and Arabs—
This is a momentous question…
The law had sent them back to Jerusalem; but years before, because they attempted to build themselves into a nation that would reach the heavens and gain control, (Genesis 11:3–4_)..God had confounded their speech and made it impossible to understand each other and place themselves over God
Now, suddenly they all understood Peter and each other. The barriers had disappeared. Their conversation was no longer “babbled” Their unity had been reaffirmed, their understanding made possible, and they were called to serve God.
Today many look upon the language of Pentecost as being able to speak in an unknown tongue; but on the Day of Pentecost, human understanding (or hearing) was the issue, not a prayer language.
Where then do we get the justification of praying in an unknown language?
Likewise the Spirit helps us in our weakness. For we do not know what to pray for as we ought; but the Spirit himself intercedes for us with groanings too deep for words. And he who searches hearts knows what is the mind of the Spirit, because the Spirit intercedes for the saints according to the will of God. (Romans 8:26-27)
On Pentecost, tongues drew the people together. Praying in the spirit causes us to pray according to the will of God, not a creation of our minds. i.e. Speaking His will into being.)