Why can't most people in churches tell you what they believe?
Most church attendees cannot state their beliefs because they have not been taught. The church has failed miserably in teaching its people, so Roman Catholics, Episcopalians, Baptists, Methodists, and others often do not really know what they believe. Paul therefore calls believers to be well-taught and well-read in the Scriptures.
Les explains that this failure is not limited to one group. He says you can ask the average Roman Catholic, Episcopalian, Baptist, or Methodist what they believe, and most cannot tell you. The reason is that nobody teaches them. Les ties this to Paul's concern that believers should not be untaught but should be a well-taught people, well read in the Scriptures. Paul does not want the Gentile believers ignorant of the mystery, the secret God kept in His own mind until He revealed it to the Apostle Paul.