We think of great events, like D-Day and the men who lived it and came home with stories, and we realize how tightly we hold on to those stories after their tellers are gone.
Irenaeus was a disciple of Polycarp who was, in turn, a disciple of John, Apostle and gospel writer, who himself was a disciple of Jesus. For that reason he jealously guarded the gift given to him.
How often are stories lost or mangled within just a few generations! How tightly must we hold to the truth, the truth that has only one source.
For in no other way could we have learned the things of God, unless our Master, existing as the Word, had become man. For no other being had the power of revealing to us the things of the Father, except His own proper Word. For what other person "knew the mind of the Lord," or who else "has become His counselor? " Again, we could have learned in no other way than by seeing our Teacher, and hearing His voice with our own ears, that, having become imitators of His works as well as doers of His words, we may have communion with Him, receiving increase from the perfect One, and from Him who is prior to all creation. We who were but lately created by the only best and good Being, by Him also who has the gift of immortality, having been formed after His likeness (predestined, according to the prescience of the Father, that we, who had as yet no existence, might come into being), and made the first-fruits of creation have received, in the times known beforehand, [the blessings of salvation] according to the ministration of the Word, who is perfect in all things, as the mighty Word, and very man, who, redeeming us by His own blood in a manner consonant to reason, gave Himself as a redemption for those who had been led into captivity.
-- Against Heresies, Book V, 1