Q: Can you sum up Stoicism in one sentence?

Not easily.  The ancient Stoics did sum up their philosophy in a single phrase: “living in agreement with nature”, which they took to be synonymous with “living in accord with virtue”.  They used other Laconic phrases as well, but Epictetus warns his students that although these are easy to memorise, people are bound to ask questions like “what’s nature?” or “what’s virtue?” and then the explanation inevitably becomes more long-winded.  Stoicism was also traditionally summed up as the philosophy that believes “virtue is the only true good” – that’s its central and most characteristic doctrine.

If someone is really struggling with these ideas you could just say that in the opening sentence of Epictetus’ Stoic Handbook he explains that Stoicism involves making a clear distinction between what is completely “up to us” and what is not, in any given situation.  Stoic Ethics and the therapy of the passions ultimately derives from that basic concept.”

~Donald Robertson, author, teacher, philosopher, REBT therapist
https://donaldrobertson.name/stoicism-faq/