Not with a capital c they weren’t, they were part of the catholic (small c, as in universal) body of Christ as are all who have ever trusted in Christ alone. And there is no way Peter, Mark, John, Matthew, Luke, and the rest of the first Christians could possibly have been part of the RCC or any organized Catholic (Capital C, as in imposter) since there was none at the time.
The Bible did not come complete with an index, telling us which books, and how many, are inspired writings and canonical or not. It was the bishops of the Catholic Church under the guidance of the Holy Spirit that sorted out and decided the canon of Sacred Scripture. The bishops were preserved from falling into error, as our Lord promised, on this important matter concerning the Holy Catholic Church. (Matt. 16:18; 28:18-20) (John 14,15, and 16) (1 Tim. 3:14-15) (Acts 15:28) They included Tobit, Baruch, Judith, Wisdom, Ecclesiasticus (Sirach), 1 Maccabees, and 2 Maccabees. Roman Catholics call these books deuterocanonical. Protestants call them Apocrypha. There are some additional passages in Daniel and Esther not found in Protestant Bibles.