Clement, a 1st-century bishop of Rome, refers to the leaders of the Corinthian church in his epistle to Corinthians as bishops and presbyters interchangeably.
Important to note that Clements purpose for writing to the Corinthians is because of the same issues Paul had addressed (Gonzalez p. 83).
The writings attributed to the apostles circulated amongst the earliest Christian communities. The Pauline epistles were circulating in collected form by the end of the 1st century AD.
It is definitely believed today that the Gospels come much later after wide circulation of Paul's text believed to have been as early as 49CE.