From The Courage to Be, by Paul Tillich, discussing the Stoics:
The larger concept of courage which includes an ethical and ontological element becomes immensely effective at the end of the ancient and the beginning of the modern world, in Stoicism and Neo-Stoicism. Both are philosophical schools alongside others, but both are at the same time more than philosophical schools. They are the way in which some of the noblest figures in later antiquity and their followers in modern times have answered the problem of existence and conquered the anxieties of fate and death. Stoicism in this sense is a basic religious attitude, whether it appears in theistic, atheistic, or transtheistic forms.
Therefore it is the only real alternative to Christianity in the Western world.
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One event especially gave the Stoics’ courage lasting power—the death of Socrates. That became for the whole ancient world both a fact and a symbol. It showed the human situation in the fact of fate and death. It showed a courage which could affirm life because it could affirm death.
From The Courage to Be, by Paul Tillich, discussing the Stoics: