Peter had spoken of the need for doing good in the face of evildoers and that could be done only by referencing “Christ the Lord as holy” (1 Peter 3:15). With the present digression Peter draws out that Christ is indeed Lord; more, “Christ the Lord” is “holy” (unique) (1 Peter 3:18). Though Jesus was horrendously abused (“suffered,” “put to death by flesh”), the demons (“spirits in prison”) were put to shame through his ascension (“proclamation” of victory). Jesus’ success in shaming the spirits behind his tormentors gives Christians courage to persevere. If he could arrive in heaven, so can the heirs of his blessed work, per 1 Peter 1:3.
19 in which he went and proclaimed to the spirits in prison,