Natura non facit saltum - nature makes no jumps - was a guiding motto for generations of evolutionists and proto-evolutionists. But Darwin encountered it in a sharp and interesting form, posed as an alternative of terrible import: nature makes no jumps, bud God does. Therefore, if we want to know whether something that interests us is of natural origin or supernatural, we must ask: did i arise gradually out of that which came before, or suddenly without any evident natural cause?
We can, of course, ask this question about anything in the natural world. We can also ask it about the very idea of God. And it was in this form that Darwin encountered the question, while a student at Cambridge. Amang the pages of his student notes survive, there are a few sheets outlining the argument of The Evidence of Christianity Derived from its Nature and Reception by John Bird Sumner, then Bishop of Salisbury, later to become Archbishop of Canterbury. ...
Sumner`s central argument rests on a simple proposition cast in a specific logical form: nature makes no jumps, therefore if something is found in the world that appears suddenly, its origins must be supernatural.... Darwin made  a chapter-by-chapter outline of Sumner`s Evidence. Among his notes there is the following passage: "When one sees a religion set up, that has no existing gives great probability to its divine origin."
In other words, sometime in his Cambridge years, 1827-30, Darwin took cognizance of the proposition that in order to show something is of natural origin it must be shown that it evolved gradually from its precursors, otherwise its origins are supernatural. This formulation of the choices open to rational men remained a leitmotif throughout his life.

Gruber Howard. (1981) Darwin on Man: A Psychological study df Scientific Creativity
University of Chicago Press, Chicago, pp 125-26