2And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting.3And there appeared unto them cloven tongues like as of fire, and it sat upon each of them.4And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
And all the people were "amazed and marvelled" at what they had seen. They had experienced the presence of the Lord, and they would never be the same. This experience that leads to transformation seems to be a theme throughout the whole Bible. An encounter, or an epiphany, with the living God just cannot leave someone unchanged. Jacob wrestling with God, Moses and the burning bush, Uzzah (killed for touching the Ark of the Covenant, which was God's presence), Paul on the road to Damascus, to name a few. Throughout the Bible, the presence of the Lord brings people to their knees. It wrecks them, and forever changes them, just like it did in Acts 2. Maybe it's not all about understanding God, but experiencing his presence in a way that brings us to our knees in worship.
Thou wilt shew me the path of life: in thy presence is fulness of joy; at thy right hand there are pleasures for evermore. Psalm 16:11