Jesus of Nazareth, the Christ of the Gospels, may have lived and died sometime during the period 10 BCE to 20 or 30 CE. It was not until the Council of Nicaea in 325 CE, three centuries later, that an agreed creed confirming the triune interpretation was established, and not until the Synod of Hippo in 393 CE that the biblical cannon of the New Testament was agreed - some three and a half centuries after the events that they purport to record. (Not exactly contemporaneous recording!) During the intervening period many differing interpretations and versions of “the life of Jesus” and its significance competed with each other, ranging from those that identified Jesus as fully divine to those that identified him as fully human, from those who saw the teachings in the light of lapsarian and eschatological thought and those who took them to be more immanentist or gnostic.
It should also be born in mind that the loosing sides in the many disagreements and debates during this formative age would run the risk of being declared 'heretical' and would be in considerable danger. A fact which may have lead to the destruction of many alternative gospels and other writings. In these theological battles of the early church the winner took all and and all evidence of the loser's thought was expurgated.
The writers and subsequent editors and translators of the competing gospels and other writings went to some lengths to tie in the stories they were telling with the Jewish belief in a messiah. There was a need in marrying the two halves of the evolving bible, the Jewish written Old Testament and the new writings of the New Testament – to make the new mesh in with the old. The messiah was the bridge that allowed that linkage to be made.
With regard to the doctrine of penal substitution, it might be interesting and informative to consider the influence of both Zoroastrian and of Pagan belief on its development; Zoroastrian belief being millennial and dualistic and Pagan being sacrificial and resurrectional (the enactment of death to create re-birth and regeneration, often the death and subsequent resurrection of a god or demi-god, or of their human incarnations or substitutes).
Judaism may well be the father of Christianity, but Paganism is the mother and Zoroastrianism its great uncle.