Edit:
There is still a decent amount of variability in views even among Protestants that believe Catholics are not saved. Most of these views, however, boil down to aspects of Catholicism that Protestants see as directly contradicting scripture, especially those that pertain to salvation. This includes
- Praying to anything other than Christ or the Father
- Belief that salvation is applied to believers by the church
- Belief that any form of ritual or act is needed to ensure salvation
- Belief that works or human effort need to be added to salvation
- Belief in purgatory
Their view is that belief in any one of these things indicates you do not believe in the salvation that the bible teaches. If you do not believe in the salvation that the bible teaches, then you are not saved.
Probably the most common objection is with regard to works. Protestants will point out that Eph 2:8-9 (and other passages) explicitly mention that salvation is not through human works or effort, and that the Bible goes to extensive lengths to emphasize this as a core aspect of both salvation and saving faith. Since Catholics do not believe this, Protestants view this as rejecting a critical aspect of faith and so they cannot possibly be saved.
Some Protestants even go so far as believing Catholicism is closer to Islam than it is to true Christianity. This is because both Islam and Catholicism teach that salvation is a combination of belief, faith, good works, church attendance, and ritual observance, and that it is impossible to know if you have done enough to merit salvation until you have died and your soul is weighed/judged. This is in stark contrast to what Protestants view as true Christianity: faith alone being sufficient and an absolute guarantee of salvation regardless of your past, present, or future deeds.
