What is a Christian?
At the most basic level, a Christian is a disciple of Jesus Christ. And yet many Christians today couldn't tell you what a disciple of Jesus Christ is, or would even think of themselves as disciples. And yet in the Great Commission, Jesus specifically called us to make disciples. Everything else is secondary.
The Cost of Cheap Grace is an extended, sweeping, bold, and bracing call to repentance for where we've let secondary things subvert our commitment to discipleship, and a compelling vision for discipleship as the basis of the gospel in all its world-changing, subversive power.