I've previously written that Zen was the religion of the samurai and Stoicism was the philosophy of the centurion. As with most generalizations, it is not strictly true. The first assertion is certainly more correct than the second. Japan was a more homogeneous society. Part of what made Rome great was its tendency to annex its conquered cultures. Only in rare exceptions did they adopt the more universal practice of destroying other nations. From its founding in the mists of 700-800 BC to its slide into political and military irrelevance around 400 A.D., the Romans conquered villages, towns, tribes, nations, and empires as they steadily grew.
Along with this, they adopted the gods, mores, and cultures of the conquered. Their standard practice was to carry the captured gods of the lands they conquered back to their Pantheon and add them to their polytheistic deities. They offered the leadership of conquered lands Roman Citizenship, which became the most coveted title of the Mediterranean and Gothic worlds. Rome was a genuinely globalist nation.
The religion of the centurion was, at various times, Greco-Roman polytheism, stoicism, Mithraism, various Middle-Eastern mystery cults, and, finally, Christianity. It is probably more correct to say that Stoicism was the prevalent philosophy of the Roman intelligentsia in the First and Second Centuries A.D. What captured my imagination, however, was Marcus Aurelius penning The Meditations while campaigning against various tribes in the East. His small book of stoic self-advice and admonitions became the most famous book of Stoic philosophy and a classic of Western literature. Writing the Second Century A.D., a time of Rome's zenith, Marcus was one of the Empire's finest Emperors, later known as the Five Good Emperors of the Nerva-Antonine dynasty. Just a little before him, the first of the great Roman thinkers was Epictetus, whom Marcus often quotes. Ironically, he was born into slavery, though his intellect eventually won him freedom. Epictetus taught and wrote extensively though none of his work survives history. What we know of his philosophy comes from a pupil, Arrian.
The most famous of Arrian's works is a small compendium of Epictetus's thoughts called The Enchiridion. It, along with The Meditations, form the basis for what we know as Roman Stoic ethics and behavior. I first read in 1976 in a class on Classical Philosophy. It was my last semester before going on active duty in the U.S. Marines. The Enchiridion, the Marine experience, and, The Meditations would mold my adult view of life. Both books are unstructured aphorisms, which are mostly read singly for comprehension. The first is more formal and didactic. It is a teacher offering advice to pupils.
Conversely, Marcus never expected his notes to be made public. They are private remarks made to himself to steady his mind during a life of brutal campaigning and bare-knuckle Roman intrigue. While Marcus wrote a manual of mental behavior for himself, Epictetus' teachings covered a range of intellectual disciplines. From these, Arrian created a manual on worldview and ethics.