The call of Isaiah focused on his lips and the lips of the people. This focus was because of the people's false speaking and false living. Isaiah's call was to confront the people about their hypocrisy of saying I believe and then doing the opposite.
In Isaiah's vision, one of the seraphs flew to the altar, took a burning coal and touched Isaiah's lips. (Isaiah 6:6-7) It is interesting to observe the symbol of heat, fire or flames as it accompanies the experience of the divine. For Moses, it was the burning bush, Isaiah with a hot coal to his lips, and Jeremiah had the Word burning in his bones. Have you ever heard the words spoken in your church, I am on fire for the lord? Now you know the brief history of this phrase.