Greek philosopher Democritus came up with the concept of atoms as the most basic building block of matter 400 B.C.  The word atom means indivisible.

Atom - eternal and invisible; absolutely small, so small that their size cannot be diminished; absolutely full and incompressible, they are without pores
and entirely fill the space they occupy;

Until the 19th century people believed that atoms are indivisible and the world is made of them.