Is Roman Catholicism the same thing as Christianity?

I don't think so. Christianity is not just a religion. It is a lifestyle, a relationship with God. It is following Christ...admitting you are a sinner and turning away from your sin...believing Jesus Christ died on the cross to save you from your sins and to give you eternal life. Being a Christian does not mean being a member of the Roman Catholic Church.  It means being a member of the body of Christ which is accomplished by faith and trust in Jesus alone for the forgiveness of your sins.  It means that you do not add your works to His work.  Sincerity doesn't forgive sins.  Membership in a church doesn't forgive sins.  Doing works of penance doesn't forgive sins. Praying to Mary doesn't forgive sins.  Forgiveness is received in the faithful trust and acceptance of Jesus' sacrifice on the cross. No one can say whether a Roman Catholic is truly a Christian or not since we cannot know people's hearts.

If you want to compare the roman catholicism religion and christian religion, then I would say that they are different a bit. The difference is about their doctrine. Yes they worship the same God, but there are some differences about their belief.

One contributor says that originally there was only one church. I think from the very beginning Christians were arguing with one another. It is recorded in the Acts of the Apostles where there is an argument about whether or not one needed to become a Jew before becoming a Christian. The very first Christians believed in the immanence of the Second Coming. When it did not come they had to change their views. The doctrines of mainstream Christianity are very much the result of historical accident and to do with power struggles in the Roman Empire. Indeed, but for chance, Mithraism could easily have become the religion of the Empire with  the figure of a bull, rather than a cross, featuring in Western culture. The doctrines and practices of Christianity may be described as a mixture of Jewish ethics, late Greek philosophy, Middle Eastern mystery religions and Roman law. It is likely that Jesus was a reforming rabbi who wanted to stir up the Jews at the time of the Roman occupation. The Romans were very tolerant in matters of religion and it seems unlikely that they would have condemned Jesus for blasphemy. Much more likely that he was involved in rebellion. There are hints of this in the New Testament. Jesus driving the moneychangers out of the temple sounds like a toned down version of an assault of some kind. What was St Peter, a fisherman, doing with a sword in the garden of Getsemane? Why was Judas needed to identify Jesus who only a few days before had apparently ridden into Jerusalem in triumph?

G'day Forbes,
You are right, Christians have been debating aspects of their faith since the beginning. Originally the Christians were seen as a Jewish sect, because the very first Christians were all Jews, so it is natural and to be expected that they should discuss whether or not someone outside the Jewish faith should become a Jew before becoming a Christian.
The first Christians were continually discovering new things about their faith and their God, much of it was understood through discussion. We should all be open to learn new things; none of us know everything or are right about everything.
Are the doctrines of Christianity the result of historical accident or divine design?
Depends on how you want to see it I guess. Fact is, Jesus appeared on earth at exactly the right time and in exactly the right place for Christianity to spread throughout the world and still be a major influence even now 2000 years on. What of Mithraism?
Yes, many of the principles of the Christian faith can be seen in many other cultures. The universal concept of right and wrong, conscience, life after death, eternity and the need to do something to make up for what we have done wrong. (As I said in my first post, we can never do that, which is why Jesus had to.) These basic concepts are universal even among remote and isolated cultures. I see this as evidence of the character of God and his existence.
The Romans did see Jesus as a threat, they were concerned that he may lead a rebellion, it seems the Jews were expecting that of him also. But there is nothing to indicate that this was his intention, either in biblical writings or other documents found.
Jesus driving the moneychangers out of the temple was an event that happened within the Jewish community, in fact within the temple, how could this possibly be viewed as rebellion against the Romans?
Simon Peter had not been a fisherman for about three years, I do not know why he had a sword, could be he was expecting trouble. I am guessing that although many people would have seen Jesus riding into Jerusalem, obviously the Roman guards that were there at Gethsemane were not among them, the Romans did not hang out with Jews.

All this is just details, we can get tangled up in this sort of discussion indefinately and it achieves nothing if our minds are set and not open.
Maeve is right, in the end what it comes down to is this, "Christianity is not just a religion. It is a lifestyle, a relationship with God."