‘Who his own self bore our sins in his body on the tree, that we, having died to sins, might live to righteousness, by whose stripes you were healed.'
And here we learn why Jesus made no claims of innocence. It was because He was there as the bearer of sin. He knew that what He was receiving was the just punishment, not for His own sin, but for the sins of others (compare 1 Peter 3:18). And He was willing to suffer for their sake. So Peter fully knew that he could not stop at the fact that Jesus had suffered for righteousness' sake, because he was deeply aware that Jesus' death had accomplished far more than that. And so he adds the crucial element, ‘Who his own self bore our sins in his body on the tree.' In other words, by being crucified He was acting as a sacrifice made on our behalf in order to ‘bear' our sins, that is, in order to take the consequences of our sin upon Himself (compare Isaiah 53:12). He was being offered as a ransom for many (1 Peter 1:18-19; Mark 10:45). His blood was being shed as an atonement and propitiation (Romans 3:24-25) so that we might be purified by being sprinkled with it (1 Peter 1:2). And He was being made a curse for us by hanging on a tree (Galatians 3:13).
This concept of His being offered as a sacrifice is partly taken from Isaiah 53:10 where the Servant was to be made a guilt offering for sin, and from Isaiah 53:12 where He was to be ‘numbered with the transgressors'. It will also be noted how similar the language is to that found in Peter's sermons in Acts 5:30; Acts 10:39 with its reference to the cross as ‘the tree', where the point is being made clear that thereby He was bearing a curse for us, because ‘cursed is he who hangs on a tree' (Deuteronomy 21:23; Galatians 3:13).
But Peter then expands on it to make clear that He was thereby also dying as our representative, so that when He died we were to be seen as ‘dying' (literally ‘departing') with Him. And he emphasises the point that as a result of our having so ‘departed' with Him in our old selves, we must now live unto righteousness. We must be obedient and Christlike. While not possibly going quite so deep as Paul does, this is very similar to Paul's teaching in Romans 6:1-11; Galatians 2:20. Compare also Ephesians 4:22-24.
He then again makes quite clear that he sees Jesus as fulfilling the role of the Suffering Servant of Isaiah for he adds, ‘by whose stripes you were healed', which is a clear citation of Isaiah 53:5. This was very much apposite in this case as the sufferings of the household servants probably largely included receiving such stripes. And the thought therefore includes the consolation that they would then receive from Christ's own stripes. He is saying, ‘do not be bitter when you receive such stripes. Remember that it is such stripes, delivered to the Innocent One, that make it possible for your souls to be healed'. Thus the overall point is that through His sufferings they have received healing of soul.
So Peter sees Jesus as being offered as a sacrifice for sin, as dying as our representative and substitute, as being cursed for us (on the tree), and as being the Suffering Servant on Whom was laid all the iniquities of God's true people, with the result that spiritual healing is made available through His blood, something which he now stresses.