In a PhD dissertation on 1 Pet 3:18-22, Chad Pierce wrote the following about the salvific efficacy of water baptism:
The Efficacy of Baptism
This salvation by God through the instrument of baptism serves two main purposes in 1 Peter. First, baptism involved the individual and communal transformation in the life of the believer. Those who had formerly lived in ignorance (1:14, 18; 2:11; 4:3, 4) were now welcomed, through baptism, into a new community (1:2; 1:3-2:10; 2:17; 5:9). This new life involved in a new moral code of obedience which is spelled out throughout the letter. The efficacy of baptism, according to verse 21, is due to the resurrection of Christ from the dead. Baptism is what identifies the individual as belonging to Christ. This is described in 1 Peter as having new birth (1:3, 23; 2:2; Jn 3:1-6; Titus 3:5). Through baptism, people become children of God (1:14). Therefore, as people who had been reborn into a new community, welcomed into a new household (2:4-10), and became children of God (1:14) through baptism, they now shared in the sufferings and glories of Christ. Furthermore, Christ, as the example, served to offer hope for those who were initiated into his new community.
Baptism as the Initiation into a New Community
There appears to be a relationship between 1 Pet 3:21 and 1:3-7. They are connected through the resurrection of Jesus from the dead. The salvation offered in baptism echoes the message that Christians have been born again through Christ's resurrection (συ σαρκος αποθεσις ρυπου, αλλα συνειδησεως αγαθης επερωτημα εις Θεον δι' αναστεσεως 'Ιησου Χριστου). The imagery of rebirth/baptism and Christ's resurrection allow for other promises for new believers to be evaluated as a benefit of baptism as well. First, the readers are assured of a imperishable, undefiled, and unfading inheritance that is kept in heaven for them (κληρονουιαν αφθαρτον και αμιαντον και αμαραντον, τετηρημενην εν ουρανοις εις υμας). In this verse, those who have been baptized into the community and experienced rebirth in Christ are promised an eschatological reward. This coheres with the theme of comfort in the midst of suffering found throughout the letter. (Chad Pierce, "Spirits and the Proclamation of Christ: 1 Peter 3:18-22 in Its Tradition-Historical and Literary Context" [PhD Diss., University of Durham, 2009], 233-34)
Further Reading