From ‘The mystical formation of Paul Tillich‘, by David Nikkel:
Tillich’s decisive experience relative to art and mysticism occurred on his last furlough of World War I, which overlapped the end of that terrible War. He had turned to studying magazines and books with classic works of art to provide some sense of hope and beauty, some link to sanity, in the midst of the despair and ugliness of the Western front. One of the works he had viewed in the trenches was Botticelli’s “Madonna with Singing Angels.”
Tillich now rushed into the Kaiser Friedrich Museum to view the original. The setting of the painting called attention to the work: it hung alone on the wall opposite the entrance. Gazing up at it, an ultimate meaning grasped Tillich. The traditional religious content (Inhalt) had nothing to do with this effect. Rather the form(s) of the colors and their spatial arrangement became the vehicle for experiencing a divine depth content (Gehalt).
Recollecting this moment for Parade magazine in 1955, Tillich wrote, “ … Beauty itself … shone through the colors of the paint as the light of day shines through the stained-glass windows of a medieval church. …. I turned away shaken.” (Note the architectural reference.) Tillich concluded, “I know now that the picture is not the greatest. I have seen greater since then. But that moment of ecstasy has never been repeated.” It constituted for Tillich a second birth that “brought vital joy and spiritual truth” to a sick soul. It also gave to him “the keys for the interpretation of human existence,” providing the basis for his theology of culture.