I understand that agnostophobia derives from an ancient Greek word, while angst apparently evolved out of Danish and German about 1942. They have a somewhat similar meaning, especially in philosophy, making me wonder if they ultimately share a common origin.
You can go ahead and close or delete this question. KarlG gave me the answer I was looking for, even if it isn't the one I was hoping for. I guess this is just an example of two words with different origins converging in meaning.