Peter wishes to explain how it is possible that people can believe in God. Peter’s readers had been enslaved to the passions of their former way of life (1 Peter 1:14, with detail in 1 Peter 4:3–4). He had written in 1 Peter 1:3–4 that God had sovereignly caused his readers to “be born again” (escape from the narrow confines of their former worldview/lifestyle) through the resurrection of Christ from the dead. With his reference in 1 Peter 1:21 to Jesus’ resurrection, he reminds his readers of the cause of their hope in God, and so explains how it was possible for them to direct their hope and confidence in God.
21 who through him are believers in God, who raised him from the dead and gave him glory, so that your faith and hope are in God.