Isaiah 6:6-7. “Then flew one of the seraphims unto me, having a live coal in his hand, which he had taken with the tongs from the altar: And he laid it upon my mouth, and said, Lo, this hath touched thy lips; and thine iniquity is take away, and thy sin purged.”
Isaiah’s contrite heart brought an immediate response from God. In his vision, a seraphim picked up a hot coal from the altar of incense, using tongs. He then brought the coal to Isaiah and, in the vision, laid the coal on Isaiah’s lips. The fire was a symbol of the purging of sin, and a cleansing that would make him able to preach the messages that God would give him.
I’m going to quote Mr. Vine again. The man just has a wonderful way of making things clear:
The whole vision and the Divine dealings were the appointed preparation for the solemn testimony he was to deliver. This was not the beginning of his witness, the occasion was a special one. If we are to engage in any particular service for the Lord, we can render it effectively only as we freshly appropriate to ourselves the efficacy of the atoning sacrifice of Christ for the cleansing of our hearts from sin. For each occasion we must come to the Throne y way of the Cross. We must come the the Mercy-Seat (Christ Himself) “that we may obtain mercy.”