John von Neumann (1903-1957) was one of the
greatest mathematicians of the 1900-2000 century,
and the inventor of the plan of the modern computer.

He said [my emphases added]:
"The calculus was the first achievement of modern
mathematics, and it is difficult to
overestimate its importance. I think it DEFINES...
the inception of modern mathematics... and it
still constitutes the greatest technical advance 
in exact thinking."

comments:
By "modern", JvN means "as opposed to ideas
developed by ancient Greeks before 200 BC."
The calculus was, thus, the greatest thing to happen to math
in 1700 years, and maybe ever.  It is pretty much regarded
as the first "real math" taught in College.  Pretty much
all the math taught in precalculus courses was known 
to the Ancient Greeks in 200 BC and once you've had calculus
you'll regard all of it as comparatively trivial.