Complex sentence: He is warning his fellow Greeks that it is not enough for them to be of Greek blood-they need a proper Greek (that is, Athenian) education as well, lest their culture be overtaken by barbarians.
Split into simpler sentences: He is warning his fellow Greeks that it is not enough for them to be of Greek blood. They need to have a proper Greek (that is, Athenian) education as well, lest their culture is overtaken by barbarians.