Clement is, so far as I know, the first to make this distinction between Peter the Apostle, and Cephas, one of the Seventy. The reason for the invention of a second Peter in the post-apostolic age is easy to understand as resulting from the desire to do away with the conflict between two apostles. This Cephas appears frequently in later traditions and is commemorated in the Menology of Basil on December 9, and in the Armenian calendar on September 25. In the Ecclesiastical Canons he is made one of the twelve apostles, and distinguished from Peter.