I am writing to you, fathers, that you have known him who has been from the beginning. I am writing to you, young people, that you have conquered the evil one.
Commentaries treat this section with near complete unity as an exhortation to the Church, rather than providing the reason John is writing his epistle, demonstrating that the Church has understood these verses to be exhortative rather than causal.
The two sequences in 1st John 2:12-14 contain a threefold address: to children, fathers, and young men.
Clement of Alexandria (c. 150 – 215) seems to best represent how the early Church understood these personifications:
“Little children” means those whose sins have been forgiven.
“Fathers” John means those perfect people who understood everything from the very beginning and readily perceived that the Son had always existed.
“Young men” John means those who have overcome their lusts.
This consensus held through and is carefully articulated by a seventh century monk:
Andreas: The different ages here are to be understood in spiritual terms; they refer to our maturity in faith. First you must become a child and be weaned off evil. It is in this state that you must put off the weight of your old sins. Once you have done this, you can progress to the status of adolescents, when you must struggle against evil. Finally you will be deemed worthy of the deep knowledge of God which characterizes parents. This is the best and truest order of growth toward acceptance by the Father.
The apostle John is writing to remind the Church that her sins have been forgiven in Christ’s name. He is writing to exhort the fathers of the Church to fix their eyes on the one who sits on the throne. And he is writing to exhort the young men of the Church to be strong and wage the good fight!
Biblical Reasons 