In this verse we are told that "many" became sinners by Adam, but that "many" have been justified by Christ.
Granted, Paul does not use the word "pan" (meaning "all") rather he uses the word "polloi" which means "many." Contextually though, he means "all." If he doesn't then clearly he would be leaving open the possibility that some had successfully avoided original sin, which is absolutely not his intention.
The same word is used in the Gospel also when Jesus said that his life will be a ransom for "polloi." If we don't posit that meaning "all," then we automatically accept Calvinist limited atonement.
So, one's two option are as follows if one does not accept that "poloi" means contextually "all" in Romans 5:18.
1. Posit limited atonement.
2. Accept a limited spread of original sin.
3. Accept both?
None of these is of course acceptable in light of the analogia fidei.