The following is an excerpt from The MacArthur New Testament Commentary on 1 Peter 3.

For Christ also died for sins once for all, the just for the unjust, so that He might bring us to God, having been put to death in the flesh, but made alive in the spirit; in which also He went and made proclamation to the spirits now in prison, (3:18–19)

In which also refers to what occurred with His living spirit while His dead physical body lay in the tomb (concerning His burial, see Matt. 27:57–60; John 19:38–42). He went (poreuomai) denotes going from one place to another (see also v. 22, where the word is used concerning the ascension). When the text says Christ made proclamation to the spirits now in prison, it is indicating that He purposefully went to an actual place to make a triumphant announcement to captive beings before He arose on the third day.

The verb rendered made proclamation (kerusso) means that Christ “preached” or “heralded” His triumph. In the ancient world, heralds would come to town as representatives of the rulers to make public announcements or precede generals and kings in the processions celebrating military triumphs, announcing victories won in battle. This verb is not saying that Jesus went to preach the gospel, otherwise Peter would likely have used a form of the verb euangelizo (“to evangelize”). Christ went to proclaim His victory to the enemy by announcing His triumph over sin (cf. Rom. 5:18–19; 6:5–6), death (cf. Rom. 6:9–10; 1 Cor. 15:54–55), hell, demons, and Satan (cf. Gen. 3:15; Col. 2:15; Heb. 2:14; 1 John 3:8).

Christ directed His proclamation to the spirits, not human beings, otherwise he would have used psuchai (“souls”) instead of pneumasin, a word the New Testament never uses to refer to people except when qualified by a genitive (e.g., Heb. 12:23; “the spirits of the righteous”).

Ever since the fall of Satan and his demons, there has been an ongoing cosmic conflict between the angelic forces of good and evil (cf. Job 1–2; Dan. 10:13; Zech. 3:1; Eph. 6:16; Rev. 12:3–4; 16:12–14). After the devil’s apparent victory in inducing Adam and Eve (and consequently all their descendants) to fall into sin (Gen. 3:1–7; Rom. 5:12–14), God promised to the Evil One himself eventual destruction by Messiah, who would triumph with a crushing victory over him, despite suffering a minor wound from him (Gen. 3:15). Satan therefore sought to prevent this by the genocide of the Jews (cf. Est. 3:1–4:3) and the destruction of the Messianic line itself during the time of Joash (2 Chron. 22:10–12; cf. 23:3, 12–21). When all that failed, he attempted to kill the infant Messiah (Matt. 2:16–18). Thwarted at that, he tried to tempt Christ Himself to abandon His mission (Matt. 4:1–11; Luke 4:1–13). Later, Satan incited the Jewish leaders and their followers to mob action that resulted in the Lord’s crucifixion (Mark 15:6–15). The diabolical Jewish leaders even saw to it that Jesus’ tomb was guarded lest He exit the grave (Matt. 27:63–66). The demons may have been celebrating their seeming victory in the wake of Christ’s death and burial—but only to soon be profoundly and permanently disappointed when the living Christ Himself arrived. The angelic spirits Christ was to address were now in prison (phulake; an actual place of imprisonment, not merely a condition).

At the present time believers must struggle against the powers of the unbound demon forces as those forces influence them through the corrupt world system over which Satan has rule. The apostle Paul told the Ephesian church, “Our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places” (Eph. 6:12), which clearly says that the demonic hierarchy is actively and freely conducting its evil work in the world. It was not to such unbound spirits, but to the bound demons that Christ went to announce His triumph.