On Mt. Sinai the Lord told Moses to tell the people to prepare themselves for the Lord's appearance. When He came down onto the mountain, He warned them not to touch the mountain or come near it or else they would die. Even with their preparations, He would come amongst them but they could not draw near. They witnessed His appearance in fire, smoke, thunder and the blaring of horns. He only spoke to Moses.
At Pentecost in Acts, they were told to wait but God appeared without any warning and invaded each person there individually. Once more He showed up with wind, and fire. But this time it was not a mountain but a person. He filled them with His presence and they became His temples.
The Lord is saying that now He does not want us to sit back as spectators any longer taking the words of man as His words, but He desires to come upon those who wait for Him with no warning, changing their lives forever. He wants to envelope them and flow through them setting there hearts on fire with His words spoken to them specifically. No more is it to be what the pastor says about God, but what God has said and done in my life. The work of the Lord in my life is what sets me on fire to proclaim His goodness and mercy. That makes me a true witness, not a second hand source.