As Benedicta proceeded with her narrative she gained courage. She rose to her feet and stood erect, threw back her beautiful head and lifted her eyes to the heavens as if recounting her wrongs to God’s high angels and ministers of doom. She stretched forth her bare arms in gestures of so natural force and grace that I was filled with astonishment, and her unstudied words came from her lips with an eloquence of which I had never before had any conception. I dare not think it inspiration, for, God forgive us all! every word was an unconscious arraignment of Him and His Holy Church; yet surely no mortal with lips untouched by a live coal from the altar ever so spake before! In the presence of this strange and gifted being I so felt my own unworth that I had surely knelt, as before a blessed saint, but that she suddenly concluded, with a pathos that touched me to tears.
It's from Ambrose Bierce's 'Monk and Hangman's daughter' (1911).
From the above paragraph, the bold letter 'no mortal with lips untouched by a live coal from the altar ever so spake before!' seems to having something to do with Bible, but I don't understand exactly what it really means.
Many thanks in advance!