"He that cometh after me is mightier than I ... he shall baptize you with the Holy Spirit, and with fire" (Matthew 3:11)
With the coming of the Holy Spirit at Pentecost the same imagery (fire) was continued. "And there appeared unto them cloven tongues like as of fire, and it sat upon each of them" (Acts 2:3). That which came upon the disciples in that upper room was nothing less than God Himself.
To their mortal eyes He appeared as fire, and may we not safely conclude that those Scripture-taught believers knew at once what it meant? The God who had appeared to them as fire throughout all their long history was now dealing 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. The flame was the seal of a new union. They were now men and women of the Fire.
A new heart comes with regeneration, the pure heart by the baptism of the Holy Spirit and of fire. We are born into one and baptized into the other.
~A. W. Tozer~