(1 Peter 3:18-20) 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; 19 in which also He went and made proclamation to the spirits now in prison, 20 who once were disobedient, when the patience of God kept waiting in the days of Noah, during the construction of the ark, in which a few, that is, eight persons, were brought safely through the water.
Answer: Christ died once for the redemption of His elect so that sinners who are enemies of God may be reconciled to the Father being born again through the power of the Holy Spirit, Justified by the Father and Adopted as His children.
Jesus preached to those who are now in Peter's time of writing these verses in prison (Hell) through His spirit through His servant Noah preaching the Gospel during the construction of the ark. So those who perished heard the Gospel but rejected it.
In verse 21, Baptism, which corresponds to this, now saves you. Baptism is a sign and seal of God’s grace in Jesus Christ. The startling statement that baptism “saves you” shows how close is the relationship between the sign and the reality it signifies. Noah’s physical salvation through the waters of the Flood prefigured the waters of baptism and the salvation they signify. Baptism symbolizes judgment on sin in the death of Christ and then also renewal of life (Rom. 6:4). The floodwaters were judgment to the wicked, and at the same time physical salvation for the just, Noah and his family.
not as a removal of dirt from the body. Lest his readers mistakenly attribute a magical or mechanical power to the sacrament, Peter states that the means of salvation is not performance of the external rite, but what it symbolizes—union with Christ in His death and Resurrection.