This was the word from the Lord they had been longing to hear. After their sackcloth and ashes, their weeping and mourning, weakened from their fasts, and sacrifices, and prayers, they longed with every fiber of their being to hear some token of God’s love and mercy towards them.
Apollos had everything you could possibly want to be effective: he had education, talent, skill and experience. But until Apollos had the Holy Spirit, his ministry was not effective.
This sign was a reversal of God’s judgment at the Tower of Babel. God’s judgment scattered and separated those people then, now He was uniting His people in the Spirit. At Babel no one could understand anyone else, but at Pentecost everyone could understand and hear God’s praises.