"Great men" create Focal Points[0] (or, if you believe in great men, Schelling Points) around which societies and lines of history converge. It's probably equally true, though, to say that societal dynamics have within them focal points which allow certain men (and women) to become great.
What about the changes in wider Roman society that allowed for Constantine's conversion to have a large impact?
>Suppose Louis the XVI had reformed the French government.
Would those reforms have been carried out by the nobility? Would it have been enough to satisfy the peasants?
Focusing on "great men" is a far too simplistic view of history and it ignores the impact that each of us have on history as individuals.