For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit (1 Peter 3:18).
After encouraging his readers to suffer for doing good, Peter returns to Christ's suffering" "Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God." When Jesus calls us to suffer, he understands because he also suffered, and Peter therefore implies that Christ is our example in suffering (he states explicitly in 1 Peter 2:21).
But, more importantly, Christ's suffering is unique. First, his suffering is "once" for all time: no other sacrifices for sin are needed. Second, the most significant difference between Christians' suffering and Christ's is that is his is atoning: "Christ also suffered once for sins." Peter elaborated, "the righteous for the unrighteous." Jesus died for sinners, in this place. His death was substitutionary and is the basis upon which people become right with God. Third, because Christ's once-for-all suffering is redemptive, it brings "us to God."
Christ as Mediator of the new covenant, died in order to bridge the gulf between God and humanity. His was a redemptive death for us who were far away from God, bringing us to him.
Adapted from the ESV Systematic Theology Study Bible