The above question is one that gets tossed around a bit too carelessly in Protestant circles. The answers that are often given - either "Yes" or "No" - are usually stated by people who know enough about Catholicism to be dangerous but not accurate.
Quite frankly, the question is a problematic one because it is too vague. It is not precise.
A better question would be, "If a person follows the stated teachings of the Roman Catholic Church, is he really a Christian?" It could be asked this way, "If a person follows the Vatican's stated doctrine, is he saved?"
The above questions are important, but I still don't find them to be very useful. The reason is this: they deal with large groups of people instead of individuals. We are saved at the individual level, so I'd like to propose a much more useful question.
That question is this: "Is the person who I'm talking to, who is Roman Catholic, really a Christian? Is he saved?"
This question is much more useful because it causes me to have to get to know the person and have an in-depth conversation. Instead of stereotyping what he believes, this makes me listen to what he says.
If we deal with the question on an individual basis, we will find that some Roman Catholics are really Christians and some are not. Some know Jesus Christ as Lord and Savior and some do not. Some have eternal life and some do not. The key is being in Christ.
Let's be honest: if we have this same type of conversation with Protestants, we will find that some are really Christians and some are not. Some know Jesus Christ and have received the gift of eternal life, while others have not.
Please let me be clear. In this post I am not saying that I believe that the stated doctrines of the Roman Catholic Church are as biblical as the classic doctrines of Protestantism. For example, I believe in justification by faith alone by God's grace alone. I love the 5 solas. I also believe that the doctrine of the priesthood of all believers is biblical.
The point I am making is that instead of saying that someone is or isn't really a Christian based on the group he is a part of, we should actually get to know the person and discover what he really believes. If he is a Christian, then you can rejoice with him that you are brothers in Christ. If he is not, then you may have the beginnings of a friendship that will (hopefully) lead to in-depth gospel proclamation.
To sum up - we need to actually get to know people. This is true inside the church and out.
When we do this, we will find out what they really think instead of assuming something based on what a particular group believes.
(This post obviously applies to those who may be a part of orthodox Christianity: Catholics, Orthodox, and Protestants. I'm not referring here to cults such as Mormons and Jehovah's Witnesses. We should certainly befriend those in cults such as this, but we need not think they might already know Jesus as Lord.)