“For Christ suffered once…”  

When Jesus was baptized in the Jordan River, He was begotten to a new spirit life with the Holy Spirit. The next 3 1/2 years, until He was crucified, were spent daily sacrificing His will and human life to do only God’s will. Jesus faithfully obeyed and glorified His Father through His obedience unto death. Thus Jesus received a divine resurrection to immortality, honor, and glory. Revelation 5:12 (RSV), “saying with a loud voice, ‘Worthy is the Lamb who was slain, to receive power and wealth and wisdom and might and honor and glory and blessing!’”

Christ sacrificed His human life and died one time on the cross. As Paul explained in Hebrews 9:24-26 (NKJV) 24 “For Christ has not entered the holy places (the Tabernacle) made with hands, which are copies of the true, but into heaven itself, now to appear in the presence of God for us; 25 not that He should offer Himself often, as the high priest enters the Most Holy Place every year with blood of another– 26 He then would have had to suffer often since the foundation of the world; but now, once at the end of the ages, He has appeared to put away sin by the sacrifice of Himself.” 

Romans 6:10 (NKJV), “For the death that He died, He died to sin (as the ransom and sin offering) once for all (all time)”. There is never a need for future masses or sacrifices. That would make Jesus’ sacrifice insufficient to cover our sins. But His sacrifice is all sufficient.  1 John 2:2 (RSV), “and he is the expiation (atoning sacrifice) for our sins, and not for ours only but also for the sins of the whole world.”

1 Peter 3:18, “…the just for the unjust…”

The just is the perfect man Jesus Christ. Jesus died for the unjust (sinful) Adam. Romans  3:10, 23-24 (RSV), 10 “as it is written: ‘None is righteous (just), no, not one; … 23 since all have sinned and fall short of the glory of God, 24 they are justified by his grace as a gift, through the redemption which is in Christ Jesus”.

1 Peter 3:18, “… that He might bring us to God, being put to death in the flesh but made alive by the Spirit”. 

The whole purpose of Jesus sacrificing His human life was to provide an opportunity for all mankind to be restored to the perfection that Adam lost. Acts 3:20-21 (NKJV), “whom heaven must receive (or retain) until the times of restoration of all things, which God has spoken by the mouth of all His holy prophets since the world began.”