What about Universalists who claim that all mankind will die, so in Christ all mankind shall be made alive – even Hitler?
Paul does say “For as in Adam all die, so also in Christ shall all be made alive”. But Universalists read this verse to say that all the dead will be in Christ.
It needs to be said that this is just misreading. It makes no more sense than “In France everyone will learn the French national anthem” means that the whole population of the world will all live in France and sing La Marseillaise. That isn’t what “In X all…” means. Not in English, not in Greek.
Even without a knowledge of grammar and syntax it is in any case already clear from the context that Paul did not say that all men and women are in Christ. The context clearly distinguished those “fallen asleep in Christ” (18) and “who belong to Christ” (24) from those outside Christ.
17 And if Christ has not been raised, your faith is futile and you are still in your sins. 18 Then those also who have fallen asleep in Christ have perished. 19 If in Christ we have hope in this life only, we are of all people most to be pitied. 20 But in fact Christ has been raised from the dead, the firstfruits of those who have fallen asleep. 21 For as by a man came death, by a man has come also the resurrection of the dead. 22 For as in Adam all die, so also in Christ shall all be made alive. 23 But each in his own order: Christ the firstfruits, then at his coming those who belong to Christ. 24 Then comes the end, when he delivers the kingdom to God the Father after destroying every rule and every authority and power. (1 Corinthians 15:17-24)
Those outside Christ, who have fallen asleep outside Christ are not counted as “in Christ” by some change that happens after they are already gone. The dead – as per consistent Old Testament teaching: Ecclesiastes, Psalms, Isaiah 26, are dead. As even Paul says in 1 Corinthians 15:18 above.