Jeff Purswell from Sovereign Grace ministries helps us understand.
If the gospel message expands to include “discipleship in the kingdom,” then the objective nature of Christ’s work is minimized. When the gospel is redefined as a call to a social or political movement, Christ’s work is replaced with ours. When the gospel includes my response, then the ground of my assurance lies in me rather than in Christ. Indeed, anytime we shift the definition of the gospel from God’s objective accomplishment to our subjective appropriation, the rock-solid foundation of our faith is misplaced—and the glory of God in the gospel is obscured.