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 is, in effect, saying that in baptism God said to the man coming direct from heathenism: "Do you accept the terms of my service? Do you accept its privileges and promises, and do you undertake its responsibilities and its demands?" And in the act of being baptized the man answered: "Yes." 