Why is the concept of Christ's accomplished work important for Christians?

The work of Christ is central to the Christian faith; His death and resurrection mean that the warfare against sin is accomplished for His people. According to Isaiah 40:2, the church's warfare is accomplished because Christ has conquered sin, death, and the devil. This assurance becomes a source of great comfort for believers, as their struggles, though ongoing in this life, are eclipsed by the victory Christ has achieved on their behalf. His proclamation of 'It is finished' at the cross summarizes this completion of redemptive work, leading believers to live in the freedom and assurance that comes from faith in Him.