Kernel
Euclid is the figure in whom the deductive method becomes a portable institution. The Elements is the first canonical demonstration that a small set of postulates, rigorously applied, can generate an unbounded body of true statements.
Contribution
The Elements (13 books, c. 300 BCE) — axiomatic geometry. Number theory (Euclidean algorithm; the proof that the primes are infinite). The format of the modern mathematical text: definition, postulate, theorem, proof. The pedagogy survived for two millennia.
Civilization-scale significance
Newton's Principia is structured "more geometrico" — in the manner of geometry — and is therefore an Euclidean text. Spinoza's Ethics likewise. The format that Euclid invented is the format civilization uses to think with certainty, where certainty is available.