Christians in the first and second century used remarkable methods and an effective learship structure to secure the preservation, accuracy and purity of their traditions and the Gospels.
Three factors were particularly essential, each including a number of sub-factors:
The use of apostolic succession, the memorization of tradition, the writing of tradition.
Apostolic succession, refers to those disciples who were chosen and designated apostles by Jesus himself who later selected their own disciples, who in turn eventually selected their own disciples.
Hence we witness various stages in an unbroken succession that lasted over a period of almost two hundred years. Within this arrangement, the disciples was trained in the tradition, to memorize it and preserve. His duty was to differentiate between truth and falsehood, to control the transmission alongside the other successors and the successing church, including his duty to control his fellow succesors who in turn controlled him.
This was a mutual endevour and hence the Gospel tradition from the time of Jesus and onward was safeguarded.
In the writings of Irenaeus the disciple of Polycarp, the disciple of John the apostle, the disciple of Jesus, we find an early reference to such an apostolic succession in the city of Rome, namely a succession that originated with the apostle Peter:
‘...the very ancient, and universally known Church founded and organized at Rome by the two most glorious apostles, Peter and Paul...The blessed apostles, then, having founded and built up the Church, committed into the hands of Linus the office of the episcopate. Of this Linus, Paul makes mention in the Epistles to Timothy. To him succeeded Anacletus; and after him, in the third place from the apostles, Clement was allotted the bishopric. This man, as he had seen the blessed apostles, and had been conversant with them, might be said to have the preaching of the apostles still echoing [in his ears], and their traditions before his eyes. Nor was he alone [in this], for there were many still remaining who had received instructions from the apostles... To this Clement there succeeded Evaristus. Alexander followed Evaristus; then, sixth from the apostles, Sixtus was appointed; after him...’ (Irenaeus Against Heresies, Book 3, chapter 3 and verses 2-3)
What is so significant with such list among other lists, is the early witness and information of a linked chain of successors going back to the original apostles who transmitted the information from Jesus.
Furthermore, the particular reference to Clement in this passage is significant, namely that he still had the teaching of the apostles in his ears and their traditions before his eyes. Which reveals that in 90 AD the church in Rome possessed the tradition in both oral and written form.
This coheres the wordings of Papias in Asia Minor who still utilizes the oral transmission but possesses the written accounts of Matthew and Mark.