Marx didn't invent capitalism, class, or division of labor, but he made good use of all of them.
Plenty of class struggle in ancient Rome and Greece.
It's been persuasively argued that Marx didn't invent anything, he just twisted other people's ideas and stood them on their heads.
His belief actually required that capitalism be developed to its maximum extent before socialism was even possible. This theory had to be dropped in order to explain how stages of economic development could be skipped thereby allowing Lenin to make the claim he was "Marxist."