Paul Tillich was a well-known 20th Century Christian theologian. He published his “Systematic Theology” in 1951. “What is a systematic theology?” you ask. It represents the totality of one’s beliefs, from A to Z. It’s a big book. I read it.
Late in life, Tillich spent a year living in Japan. When he came back to the States, he was asked about the yearlong experience. His reply was, “I have to re-write my systematic theology.”