Rom 5:18

Rom 5:18 uses all to refer to all men being condemned. It also uses all to refer to the free gift coming upon all men to justification.

18 Therefore, as through one man’s offense judgment came to all men, resulting in condemnation, even so through one Man’s righteous act the free gift came to all men, resulting in justification of life.

If the word all referring judgment and condemnation, in the first part of this verse, really refers to all, then the same word would also refer to all men in the last part of the verse. Both uses of the word "all" are in the same context, in the same book, even the same chapter, and even the same verse. More, they are in the same sentence.

There is no reason for all to literally mean all in one case, and to not literally mean all in the other.

Since Calvinists and non-Calvinists alike accept that all men are condemned, then we can safely assume that all in this verse also refers to the free gift coming upon literally all men.

This means the atonement was for all, not limited in its scope.