There are therefore, two figures in 1 Peter 3:21: the flood, which is an Old Testament type of baptism; and water baptism, which is the New Testament picture (or the sign and seal) of salvation in the blood and Holy Spirit of Christ. The reality is salvation in Jesus Christ.
The Heidelberg Catechism elucidates: “Is then the external baptism with water the washing away of sin itself? Not at all; for the blood of Jesus Christ only, and the Holy Ghost, cleanse us from all sin” (Q&A 72). Water baptism, and its type, the flood, point to one great reality: the washing away of our sins by the blood and in the Spirit of Jesus Christ. Peter teaches this when he writes: “The like figure whereunto even baptism doth also now save us…by the resurrection of Jesus Christ” (v. 21). Peter connects salvation not to water baptism, but to the resurrection of Jesus Christ, and therefore also to the cross. There is no resurrection without the cross, for the resurrection of Jesus Christ is his bodily resurrection from the grave three days after his death.
Peter has already explained the death of Christ in verse 18: “For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God.” Christ’s death was a substitutionary death, an atoning sacrifice to satisfy God’s justice. We are unjust or unrighteous, and Christ, the just one, paid for our sins. Thus Christ died both for our benefit and in our place, and by his resurrection God proves that he is perfectly satisfied with his Son’s work of atonement.