There’s a very common idea that a lot of Christians, and even many members of the actual body of Christ, refer to, which is the idea that one is either “in Adam” or “in Christ.” They get this idea from the fact that there are multiple verses in the Bible which do refer to being “in Christ” from a spiritual perspective, and they then look at 1 Corinthians 15:22 (which says, “For as in Adam all die, even so in Christ shall all be made alive”) and assume the phrase “in Christ” in this verse means the same thing it does in the other verses that use the phrase — being a part of Christ on a spiritual level, or being spiritually identified with Christ, in other words — and also assume that this means the phrase “in Adam” in this verse must refer to actually being a part of Adam on a spiritual level as well.
The problem is, words and phrases don’t always mean the same thing every time they’re used in Scripture, and “in Christ” in this verse is no exception. Rather than referring to being a part of Christ, in this case the phrase actually just means “through Christ’s action” or “because of what Christ did.” And likewise, “in Adam” just means “through Adam’s action” or “because of what Adam did,” referring to the fact that his sin is the reason we’re all born mortal.
And this is the only passage in the entire Bible that uses the phrase “in Adam,” so there isn’t anywhere else in the Bible to get the idea that humans are spiritually a part of Adam (who is currently dead and, as such, doesn’t even exist as a conscious being at this time) from.
Of course, there are those who might want to try to point to Hebrews 7:9-10 to suggest that we were all “in Adam” before we were even born, just as the writer of Hebrews seems to have said Levi was “in Abraham’s loins” before he was born. This statement wasn’t meant to be read as absolute truth, however. You see, it’s important to note that he began the assertion with the words, “as I may so say” (at least in the KJV), which just means “one might almost say,” or “so to speak,” in 17th century language, with the Greek it’s translated from (ὡς ἔπος εἰπεῖν/“hōs epos eipon”) being an expression used in Greek literature to qualify a statement that sounds like it should be taken as absolute truth in order to de-emphasize what was being said. Simply put, the writer wasn’t saying that, when Abraham gave the tithe, Levi was also actually giving a tithe too. He was just making a metaphorical statement in order to illustrate a theological point, and wasn’t intending us to take it literally at all. Besides, even if being said to have been in someone’s loins in the way the writer of Hebrews used the expression was meant to be taken as a literal statement, none of us are still in Adam’s loins, so to speak, anymore, since we’ve now been born, so none of us would still be “in Adam” in that manner anyway.
That’s really all I have to say about the topic, but if you want to understand the reason that the phrase “in Christ” in 1 Corinthians 15:22 does have to be referring to Christ’s action by going to the cross rather than being a part of Him spiritually, I went into more detail on this in chapter 6 of my eBook, so please check that out if you aren’t aware of the reason already (although, if you aren’t already familiar with the scriptural basis for the doctrine of the salvation of all, I’d recommend starting at the beginning of the book).