I am not a Roman Catholic. In my understanding of Roman Catholicism, the term "saints" is not applied to all believers. It's applied to a subset of believers who have performed some miracle or great act.
When I look at Romans 1:7, I see:
To all those in Rome who are loved by God and called to be saints:
From this, we can logically derive:
"those people who are in Rome" AND "are loved by God" ==> can be called a saint
I suspect the condition of "being in Rome" is not a hard requirement, i.e. it's there became it's the book of Romans and Paul is writing to people in Rome.
Thus, I get the impression that in Paul's eyes, "all those who are loved by God should be called a saint."
Thus, "all believers are to be called saints."
Is this consistent with Paul's other teachings?