George Eliot was the pen name of Mary Anne Evans (1819-1890). One of the great Victorian novelists, Eliot also completed the first English translation of David Strauss' Das Leben Jesu kritisch bearbeitet (The Life of Jesus, Critically Examined). This book was highly controversial at the time of its publication due to its rejection of the divinity of Jesus and argument that the gospels should be interpreted as myths.
Fowles' quotation comes from a conversation between Eliot and F. W. H. Myers in 1873, which he recalled thus:
"Taking as her text the three words which have been used so often as the inspiring trumpet-calls of men--the words of God, Immorality, and Duty,--[she] pronounced, with terrible earnestness, how inconceivable was the first, how unbelievable the second, and yet how peremptory and absolute the third."