"And there appeared unto them cloven tongues like as of fire, and it sat upon each of them" (Acts 2:3)
Just because God cannot tell us what He is, He very often tells us what He is like, and by these "like" figures He leads our faltering minds as close as they can come to that "Light which no man can approach unto".
Through the more cumbersome medium of the intellect the soul is prepared for the moment when it can, through the operation of the Holy Spirit, know God as He is in Himself. God has a number of these similitudes to hint at His incomprehensible being, and judging from the Scriptures one would gather that His favorite similitude if fire.
In one place the Spirit speaks expressly, "For our God is a consuming fire."
With the coming of the Holy Spirit at Pentecost the same imagery was continued - for that which came upon the disciples in that upper room was nothing less than God Himself!
The God who had appeared to them as fire throughout all their long history was now dwelling in them as fire. He had moved from without to the interior of their lives. The Shekinah that had once blazed over the mercy seat now blazed on their foreheads as an external emblem of the fire that had invaded their natures.
This was Deity giving Himself to ransomed men and women.
The flame was the seal of a new union. They were now men and women of the Fire!
~A. W. Tozer~