What is the difference between "catholicism" and "catholic"? In the Apostle's Creed, the term "catholic" is used in describing the reciter's belief:


  I believe ... in the holy catholic church


This creed is used by a wide variety of denominations, and therefore I'd find it very hard to believe that it always refers to the Roman Catholic Church.

Some English translations of the Apostle's Creed use "holy Christian church" instead of "holy catholic church" - is "catholic", therefore, a synonym for "Christian"?

What is its use in the Apostle's Creed (and other documents)?
The wording of that creed has caused confusion among untold thousands of people over the past millennium and across scores of languages. For as much consternation as it has spawned, the intended meaning is really very simple.

The word catholic in the a Apostle's Creed is being used as an adjective, not a proper name! A quick English dictionary search will turn up possible meanings like the following:


  
  Including a wide variety of things; all-embracing  
  Of or including all Christians
  


The Roman capital-C Catholic church used this adjective as a proper name in their official title. In our current times, the word is almost ubiquitously associated with that proper name. The Apostles Creed, on the other hand, was very specifically not referring to the Roman Catholic church but to the church universal. This is why a clear majority of Christian traditions both Catholic and Protestant are able to stand behind the Apostles Creed as a valid representation of their faith.

The various alternate wordings you will find (such as the one you noted: "Christian" or another common alternative: "Universal") are attempts at resolving the confusion by using a less miss-understood word.

Although it would not be a good substitute in the Creeds because it has overtones of diversity rather than unity, another related term is ecumenical. To put it all in context, it would be reasonable to say that "The Apostle's Creed's description of the core tenants of our (catholic) faith rather than a specific (Catholic) tradition makes it useful in an ecumenical context."