Bernard Bolzano (1781–1848) was a Bohemian priest, philosopher and mathematician who insisted that calculus should be built on airtight logic, not hand-waving about "infinitely small" quantities. He was decades ahead of his time — and almost nobody noticed while he was alive.
Bolzano was one of the first to pin down exactly what it means for a
Bolzano's honesty got him in trouble. His outspoken sermons on peace and social justice annoyed the Austrian authorities so much that he was stripped of his professorship, placed under surveillance, and forbidden to publish. So he kept writing quietly and let much of his best mathematics sit unread for decades. When those manuscripts finally surfaced, mathematicians were amazed to find he'd anticipated ideas they thought were brand new. A man whose greatest work was basically a message in a bottle to the future.