Christ bore our sins in his body, 1 Pet 2:21-25

The suffering of Christ is certainly presented in these verses as exemplary, but this does not exhaust the meaning.  For one thing, there are clear allusions to Isa 53, and to the Suffering Servant from whose sufferings we benefit.  The ‘bearing’ of sins in Isa 53 implies the bearing of punishment.  And the echo of Isa 53:5 in ‘by his wounds you have been healed’ highlight the fact that he suffered so that we might not – substitutionary atonement indeed.  Again, Peter’s reference to Jesus bearing our sins ‘on the tree’ takes us back (as does the similar passage in Gal 3:13) to the penalty referred to in Deut 21:23.

This passage, therefore, not only sets forth Christ’s sufferings as an example to Christians who suffer unjustly, but also moves on to our ultimate hope: the atoning power of his death.

Christ died for sins once for all, 1 Pet 3:18

This verse parallels the passage just mentioned.  Peter here moves again from the exemplary value of Christ’s suffering to a statement about its saving power.  Notwithstanding their value as an example, there is a vital sense in which Christ’s sufferings are absolutely unique (‘once for all’).  Then, the phrase ‘for sins’ (or, ‘as a sin-offering’) places Christ’s death in the context of the Levitical sacrifices.  The substitutionary nature of that sacrificial death is then made quite clear: ‘the righteous for the unrighteous’.

Based on Jeffery et al, Pierced for our transgressions, 95-99.