“For as in Adam all die, so in Christ all will be made alive. ” (1 Corinthians 15:22)

Firstly, “there’s no bad news in the Good News!” (literally). Even Hell is reformatory through its “chastisements”. But Christ heals them all by “making alive” (from the “death – state”), “reconciling” and “saving all men & the world” eventually; after which, both Death & Hell are destroyed after serving its purposes!

“In Christ” means “Christ makes all alive” or equivalently, “Because of Christ, all are made alive” (not by any other ‘way’ or ‘means’)

“In Christ” does not mean “believers in Christ only” simply because then we have to put the “believers” in front of “in Christ”.

You may separate the “in Christ” verses into two classes:

(i) The first type: “in Christ” refers to the “Salvation & its inheritance to believers who trusted Him first in this life”. (not exclusivism)

(ii) The second type: “in Christ” refers to the “Salvation of the rest of men” and eventually the world too! (in stages, as each man in ‘his order’ too where believers are the ‘firstfruits’, not the only fruits of His redemption Purchase which will be “testified” in its appropriate times).
If that’s not convincing… here’s a “more universal” verse which contains “in Christ” or “in the Christ” but talks about “all things” (the Universal Set) for which “all men” is just a subset:

“in regard to the dispensation of the fullness of the times, to bring into one all things “in Christ”, both the things in the heavens, and the things upon the earth — in him;” (Ephesians 1:10)

This agrees (literally) with 1 Corinthians 15:28 where God becomes “All in All”.

If that’s still tricky as to the intent of why the “all things” are brought into “one”, here’s the reason = Reconciliation:

“and through him to reconcile the all things to himself — having made peace through the blood of his cross — through him, whether the things upon the earth, whether the things in the heavens.” (Colossians 1:20)

This “also” agrees (literally) with 1 Corinthians 15:28 where God becomes “All in All”.