I also thought when I first read this book that the "Tongue of Flame" was a biblical refrence to Acts 2. However, I soon come to know that this actually describes how the Reverend Dimmesdale speaks. He has the ability to move the congegation with his words and they listen and take to heart every word he says no matter what it is. His ability to move people with his words and the tender compassionate, yet powerful tone in his voice is what is considered the "Tongue of Flame."
My AMAZING English Teacher:))