Because Christ also suffered for sins once, the righteous for the unrighteous, that he might bring you to God; being put to death in the flesh, but made alive in the spirit;
Peter makes this point because he is encouraging Christians to endure suffering. In that the suffering and death of Christ did not terminate His existence but rather was a step to a new life in the realm of the spirit, they also could look forward to a better day beyond the sphere of the flesh.