Eph 4
8 Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men.
MATTHEW 27:50-53: “And Jesus cried out again with a loud voice, and yielded up His spirit. And behold, the veil of the temple was torn in two from top to bottom, and the earth shook; and the rocks were split, and the tombs were opened; and many bodies of the saints who had fallen asleep were raised; and coming out of the tombs after His resurrection they entered the holy city and appeared to many.”
1 PETER 3:18-19: “For Christ also died for sins once for all, the just for the unjust, in order 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.”
Now read 1 cor 15 vs 20.
"....Jesus the firstfruits of them that slept"
It is clear.
8 Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men.
MATTHEW 27:50-53: “And Jesus cried out again with a loud voice, and yielded up His spirit. And behold, the veil of the temple was torn in two from top to bottom, and the earth shook; and the rocks were split, and the tombs were opened; and many bodies of the saints who had fallen asleep were raised; and coming out of the tombs after His resurrection they entered the holy city and appeared to many.”
1 PETER 3:18-19: “For Christ also died for sins once for all, the just for the unjust, in order 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.”
Now read 1 cor 15 vs 20.
"....Jesus the firstfruits of them that slept"
It is clear.
When a believer in Christ dies, their spirit departs from the body and goes to be in the presence of the Lord. They are looking forward to the resurrection from that side. Prior to Christ, when an OT saint died, their spirits departed and wend down into the comfort area of Sheol/Hades. This is revealed in the event of the rich man and Lazarus. That obviously changed after the Lord's resurrection, because Paul says to be present in the body is to be absent from the Lord and to be absent from the body is to be in the presence of the Lord. In another scripture, he also says that he was torn between the two, wanting his spirit to depart and be with the Lord, which is better by far or to remain in the body which was better for the strengthening of the Corinthians faith.