What are the oldest known Christian texts that are not part of scripture, or considered apocryphal text?
The problem is, most documentation of the early Christian Church has been deliberately destroyed.
From my answer to Why is there no archaeological evidence that Christians existed for 200 years after 70 AD? - Christianity Stack Exchange:
The lack of evidence of early Christianity has been noted by many scholars.
Edward Gibbon, in The History of the Decline and Fall of the Roman Empire, said :"The scanty and suspicious materials of ecclesiastical history seldom enable us to dispel the dark cloud that hangs over the first age of the church.".
William Fitzgerald, in Lectures on Ecclesiastical History said: "Over this period of transition, which immediately succeeds upon the era properly called apostolic, great obscurity hangs ...".
Samuel G. Green in A Handbook of Church History said: "The thirty years which followed the close of the New Testament Canon and the destruction of Jerusalem are in truth the most obscure in the history of the Church. When we emerge in the second century we are, to a great extent, in a changed world.".
William J. McGlothlin, in The Course of Christian History said: "But Christianity itself had been in [the] process of transformation as it progressed and at the close of the period was in many respects quite different from the apostolic Christianity.".
Jesse Lyman Hurlbut, in _Story of the Christian Church, said:"For fifty years after Paul’s life, a curtain hangs over the Church, through which we vainly strive to look; and when at last it rises, about 129 A.D. with the writings of the earliest Church Fathers, we find a Church in many ways very different from that in the days of Peter and Paul.".
Other historians make similar comments about the lack of historical material from that period, and how, after a few centuries, suddenly Christianity is seen to be flourishing, with no evidence of how it got that way.
Whatever happened, it was not a smooth transition.
It is beyond the AD 70-220 range of this question, but the Council of Nicaea provides a good idea of what the process must have been like.
Describing the period immediately after the Council, historian Will Durant wrote, "Probably more Christians were slaughtered by Christians in these two years (342-3) than by all the persecutions of Christians by pagans in the history of Rome" (The Story of Civilization).
The Holy Roman Empire's version of Christianity was very different from the original Christianity spread by the Apostles. Once "Christianity" became the official religion of Rome, anyone practising anything resembling original Christianity was called a Judaizer and persecuted as a heretic.
It is hardly surprising that under such circumstances, any historical records that contradicted the new official version would have been suppressed and destroyed.
One can still make a reasonable estimate of what those documents would have said though, by comparing the state of the Church in A.D. 150 with that of a century before.
What caused the Church to change so much during that time will be what the missing documentation must have described.