After the day of Pentecost 3,012 men and women were born again.  They witnessed the great miracle of hearing and seeing the apostles speak in their own tongues the wonderful works of God.

After the lame man was healed (5,000 believed) the religious leaders held Peter and John captive and threatened them to not speak or teach in the name of Jesus.
 
But Peter and John answered and said we cannot but speak the things which we have seen and heard. (Acts 4:20)
We have all heard and have seen the power of God and deliverance in the name of Jesus Christ.  Now we must be witnesses of Him.
acts420 as for us we cannot help speaking about what we have seen and heard NIV 18204 4 wide