AS IT IS WRITTEN . . .
Peter's name, which is the first word in the longer of his two epistles, is immediately qualified by the noun "apostle." The absence of the definite article with this noun prompts most English translations to insert the indefinite article, "an apostle," indicating that Peter was one of a group.
While this is grammatically correct—nor, given the difficulty of translating from one idiom to another, can I think of an obvious alternative—this rendering can be misleading. Although it is true that Peter was one of a group, the absence of the definite article in that first verse implies something . . .