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Matthew also issued a written Gospel among the Hebrews in their own dialect, while Peter and Paul were preaching at Rome, and laying the foundations of the Church. After their departure, Mark, the disciple and interpreter of Peter, did also hand down to us in writing what had been preached by Peter. Luke also, the companion of Paul, recorded in a book the Gospel preached by him. Afterwards, John, the disciple of the Lord, who also had leaned upon His breast, did himself publish a Gospel during his residence at Ephesus in Asia. (Adversus Haereses BIII, C1)

Matthew also issued a written Gospel among the Hebrews in their own dialect, while Peter and Paul were preaching at Rome, and laying the foundations of the Church. After their departure, Mark, the disciple and interpreter of Peter, did also hand down to us in writing what had been preached by Peter. Luke also, the companion of Paul, recorded in a book the Gospel preached by him. Afterwards, John, the disciple of the Lord, who also had leaned upon His breast, did himself publish a Gospel during his residence at Ephesus in Asia. (Adversus Haereses BIII, C1)

Since having an idea of when St. Peter preached and when he died would help in outlining dates for the first Gospel, and also the others, we can investigate when this took place. St. Jerome tells us on St. Peter:

Since having an idea of when St. Peter preached and when he died would help in outlining dates for the first Gospel, and also the others, we can investigate when this took place. St. Jerome tells us on St. Peter: