A more careful and critical look at the context of Acts 28 reveals that the message formerly sent to the Jews was then sent to the Gentiles. Further, that Paul was formerly preaching the kingdom of God with content from Moses and the prophets which Luke does not explicitly modify and informs us that in the hired house after Acts 28:28 Paul continued to preach the kingdom of God. The salvation of God ("it") was to be preached to the gentiles and as Paul believed (indicative mood) they would hear it! Paul later would need to declare " all they which are in Asia be turned away from me; of whom are Phygellus and Hermogenes. (2 Tim. 1:15 KJV)".
The Mystery gets into Acts 28 by reading it into the context -- otherwise it simply is NOT there! The study of Acts 29 is about refining our understanding of the close of the book of Acts and helping all who have ears to hear to perfect their understanding.