The Gospel is not just Christianity 101. It's not just entry level information.
Call me slow, but when I first encountered the idea that believers ought to be preaching the Gospel to each other -- and to themselves! -- in (Dietrich Bonhoeffer's Life Together), it was like a lightbulb switched on in my head at the same time a sucker punch rocked my gut. What a transforming truth that was.
Some of us have come a long way through guilt trips and tours of techniques and alliterative outlines and workbook curriculum designed for maximum holiness, through cult and kitsch, through hoakum and hooha, and only relatively late in our Christian walk do we realize that what we need today and tomorrow is what we needed that blessed day Jesus "came into our hearts." We needed -- and still need -- the Gospel.