Unknown Direction

"You will receive ability and you will become my witnesses... (the ability to show evidence)"

"They were all filled with the Holy Spirit and began to speak in other (unknown) languages as the Spirit gave them the ability to."

When they were filled with the Holy Spirit, they began to see and understand what the Holy Spirit wanted - His perspective on things, what the Spirit was longing for and thought was worth paying attention to. Before they would have their own view, their own priorities.

They were so consumed with this new view of the world around them that they lost all greed and ambition and "with one heart and purpose" they pressed ahead eagerly for what the Spirit was hoping for.

Tongues of fire divided and rested on them - an echo of the Tower of Babel

"May they be one, as we are one... and so the world will KNOW (have the evidence) that you have loved them" - (prayer near the end of John)

The tongue is the steering wheel that directs the whole body and the direction of our entire life, as explained in James.

When the Spirit gave them ability to speak in "new tongues" it wasn't just saying things in an unknown language. He was enabling them to steer their whole life and heart in a direction far beyond normal human reach or understanding... and also, to live in a UNITY of purpose and heart with others in a way that we could never even begin to reach on our own.

However because of this extraordinary ability given by the Holy Spirit himself, there was one mind, one purpose among so many - all working together became evidence of His love.

Peter also now had the ability to explain what the Holy Spirit wanted, so that 3000 random pilgrims from many places all became so cut to the heart by Peter's off the cuff speech that they all cried out in desperation, "How can we be saved??"