So what did Paul mean when he said "As in Adam all die, so also in Christ all will be made alive"?
In the context, clearly "in Christ" is the keyword. All that sleep in Christ will be made alive in Christ.
1Co 15:17-22 KJV
17 And if Christ be not raised, your faith is vain; ye are yet in your sins. 18 Then they also which are fallen asleep in Christ are perished. 19 If in this life only we have hope in Christ, we are of all men most miserable. 20 But now is Christ risen from the dead, and become the firstfruits of them that slept. 21 For since by man came death, by man came also the resurrection of the dead. 22 For as in Adam all die, even so in Christ shall all be made alive.
It would be far-fetched and contrary to other Paul teachings to think that everyone is in Christ and therefore shall be made alive, based on that text. On the contrary Paul is very clear that some people are not in Christ and therefore won't benefit from his gifts, they won't live.
Rom 8:6-14 KJV
6 For to be carnally minded is death; but to be spiritually minded is life and peace. 7 Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be. 8 So then they that are in the flesh cannot please God. 9 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. 10 And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness. 11 But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you. 12 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh. 13 For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live. 14 For as many as are led by the Spirit of God, they are the sons of God.
This text is very clear that we have the choice between being spiritually minded, be of Christ's, be in Spirit, be led by the Spirit, and therefore obtain eternal life, or be in the flesh, be carnally minded, live after the flesh, and die.
It is a choice to make. We won't live forever if we live after the flesh 🙏
1Th 4:16, 17 KJV For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
Therefore, those that are sleep in Christ are those that are risen again at that time, and they will live for ever "with the Lord", none of them will suffer eternally in hell.