First, let's consult the dictionary:
- Christian: a follower of Jesus Christ and his teachings
- Catholic: a member of the Roman Catholic Church
A similar situation may prevail among members or attenders of other Christian churches. For instance, suppose my parents are Baptists and they have taken me to a Baptist church all my life, starting when I was an infant, and my primary community is with the members of this church. I might well consider myself a Baptist, even if I don't believe all the things taught in my church.
Now, in contrast to the definition of "Catholic" which depends on institutional membership, the definition of "Christian" is rooted in the individual's beliefs and actions. A Christian follows Christ and follows his teachings. One could be a Christian all alone on a desert island, never having set foot in a church, if he believed in Christ Jesus.
So the terms "Catholic" and "Christian" are really speaking to two orthogonal characteristics. One deals with membership in an institution, the other deals with personal beliefs and allegiance. Thus, a person could be both a Catholic and a Christian, or be a Catholic but not a Christian (as in the case of a person who is associated with the RCC but doesn't believe), or a Christian but not a Catholic (that is, a person who believes in Christ but isn't associated with the RCC).
In the U.S., historically there have been more Christians who are not Catholics than Christians who are Catholics. Most of these non-Catholic Christians were and are Protestants:
- Protestant: a member or follower of any of the Western Christian churches that are separate from the Roman Catholic Church and follow the principles of the Reformation, including the Baptist, Presbyterian, and Lutheran churches.
How does this happen? Suppose I grow up going to a Protestant church that isn't affiliated with any particular denomination (non-denominational or inter-denominational). My parents are Protestant Christians, but I never hear them use the term Protestant; they just call themselves Christians. I may get the impression that what makes a Christian is having Christian parents, or just going to my church, or at least my kind of church--that is, I may think the definition of "Christian" is "a member of a (Christian) church"--and maybe even think that I can be a Christian without believing in Christ or actively pursuing his teachings.
And so there are competing ideas of what "Christian" means floating around in people's minds. But the meaning, the definition, the essence of being a Christian is following Jesus, and it has nothing to do with membership in a human organization. Meanwhile, being a member of the Catholic church makes you a Catholic regardless of whether you follow Jesus.
Of course, the Catholic church is a Christian church, so presumably the system for becoming a member is intended to ensure that its members are indeed following Jesus. That is, from a theological perspective, Catholic should imply Christian: follower of Christ. Catholic and Protestant Christianity do differ, in both theology and implementation, but they agree far more. They have Christ in common (plus everything in the Nicene Creed)!
A last note: The definition of Christian doesn't specify some key things. What does it mean to follow Jesus Christ? What did he teach? Without answers to these questions, it is impossible to know, concretely, what it means to live as a Christian and think as a Christian. I'll address those questions, as well as some of the differences between Catholic and Protestant theology, in part 2.