How do we know that Christ's sufferings were sufficient?
Answered in 1 source
Christ's sufferings were sufficient because His sacrifice on the cross completely satisfied the demands of God's justice for the redemption of sinners.
The sufficiency of Christ's sufferings is rooted in the doctrine of substitutionary atonement, which tells us that Jesus bore the full wrath of God on our behalf. Paul made it clear in his teachings that Christ's redemptive work is complete and perfected; He has perfected forever those who are sanctified (Hebrews 10:14). The idea that additional sufferings from believers could add to or complete Christ's work undermines the grace of God. Instead, believers are united with Christ, and while they will suffer for their faith, it does not detract from Christ’s completed work of redemption.
Commentary