The gospel of the Kingdom

No state can be completely secular in the sense that those who exercise power have no beliefs about what is true and no commitments to what they believe to be right. It is the duty of the church to ask what those beliefs and commitments are and to expose them in the light of the gospel. There is no genuinely missionary encounter of the gospel with our culture unless this happens. Here we must face frankly the distortion of the gospel that is perpetrated in a great deal that passes for missionary encounter. A preaching of the gospel that calls men and women to accept Jesus as Savior but does not make it clear that discipleship means commitment to a vision of society radically different from that which controls our public life today must be condemned as false. (Foolishness to the Greeks p132, emphasis mine)

Preaching the gospel means preaching the kingdom of God, which is 'a vision of society radically different from what controls our public life today.'