Prior to the writing of Mark, the Christians utilized a oral transmission, which today is included in the four Gospels. However, memorization in the first, second and third century church continued to utilize the oral transmission alongside the written transmission.
The early church father Ireneaus (120-190 AD), the disciple of Polycarp (70-150 AD) who himself was a disciple of John the apostle (died 90/95) the disciple of Jesus, records a number of details relating to memorization and oral transmission. In his Against Heresies, Book 3, chapter 3 and verses 2-3 he records from the line of Roman successors and narrators:
‘...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...’
Hence Clement of Rome the bishop of Rome and an apostolic successor was in position to memorize and recite the apostolic tradition.
Irenaeus himself records in the Fragments of the Lost Sayings of Irenaeus, chapter 2, how he sat under the influence of Polycarp the successor of John the apostle and memorized the traditions:
‘...I then listened to them attentively, and treasured them up not on paper, but in my heart; and I am continually, by God's grace, revolving these things accurately in my mind’.
Indeed Irenaeus records in Fragments of the Lost Sayings of Irenaeus, chapter 2 how Polycarp had engaged in memorization from the apostles, those who were eyewitnesses and hearers of Jesus:
‘...and how he would call their words to remembrance. Whatsoever things he had heard from them respecting the Lord, both with regard to His miracls and His teaching, Polycarp having thus received [information] from the eye-witnesses of the Word of life, would recount them all...’
We read here of two lines of successors, both lines leading us back to the apostles and then to Jesus, of which both lines of successors are engaged in memorization.
Of similar lines of successors we could refers to Papias who memorized the ‘The Living and Abiding Word’ (that is information transmitted by an eyewitnesses of the account and under his control) from John the Elder and Eyewitnesses and disciple of Jesus (Eusebius, The History of the Church Book 3, chapter 39).
Irenaeus confirms in Irenaeus in his Against Heresies, Book 3, chapter 4, verse 1, that in 180 AD the oral transmission is still so intact within the church, that even with no Bible the oral preservation of the Gospel would be sufficient to preserve the entire tradition:
‘For how should it be if the apostles themselves had not left us writings? Would it not be necessary, [in that case,] to follow the course of the tradition which they handed down to those to whom they did commit the Churches?’