The sermon focuses on Paul's imprisonment and his unique calling to preach to the Gentiles, as described in Ephesians 3. The pastor explains how Paul reveals the mystery that Gentiles can now come directly to God through Jesus Christ, without first becoming Jewish - a concept that was revolutionary at the time. The sermon also explores how God often calls people to serve in unexpected ways, as demonstrated by sending Peter to the Jews and Paul to the Gentiles, despite their backgrounds suggesting the opposite assignments would have made more sense.