Which of Paul's letters use the word 'mystery'?
According to the concordance cited, the word 'mystery' appears throughout Paul's epistles, identified as Romans through Hebrews. The context does not enumerate individual letters; it treats that entire Pauline collection as the range where the term is used, contrasting it with Matthew 13's mysteries of the kingdom.
Les explains that he placed the mysteries on the board according to his concordance, listing all the times the word is used in Paul's epistles, Romans through Hebrews. He says they will chase them down during the half hour to see how often Paul refers to the mysteries. He calls it a unique term. He also notes that the word appears in Matthew 13 when Jesus spoke of the mysteries of the kingdom, but that was a totally different setting. Throughout Paul's epistles, Les says, Paul constantly comes back to this term, the mystery.