Mystery religions were very popular in the 1st century Greek world and Christianity is essentially a Jewish mystery religion. 'Mystery' literally meaning 'secret knowledge'. The story of Jesus telling a parable and then secretly telling the apostles that there is a hidden meaning is itself a parable with the author of the gospel saying that the entire gospel is filled with hidden meanings to be found.
It's terrible way to spread a message and inevitably ends with people inventing their own meanings and losing what the authors originally intended to convey.
Truman became president on April,12 1945. He was given a full briefing on the Manhattan project on April 24,1945. That doesn't seem like he was being kept in the dark.