I learned this after forty years of living.
I didn't hear about it as a preacher's kid growing up, though I would assume my dad knew it. I didn't hear about it as a pastor's wife, which I was for almost 13 years - until the nonsacramental marriage ended.
I didn't hear about it as a Protestant at all.
I heard it for the first time when I became Catholic.
Why were all those people from all those different countries gathered in Jerusalem that day? Because the day of Pentecost (the day the Catholic Church was born and the Holy Spirit came down) was a Jewish holiday.
Like Easter, which is tied to the Jewish Passover, Pentecost is tied to the Jewish Feast of Weeks. Shavuot. It means 7 weeks. "A week of weeks." It falls 49 days after the Jewish Passover. And so, all those people were on a pilgrimage to Jerusalem.
And they all heard in their own languages about the Messiah who had come and died 50 days earlier - Pentecost (50). That is the day the Holy Spirit came - how perfect!