I have written to you, little children, because you know the Father. I have written to you, fathers, because you know him who is from the beginning. I have written to you, young men, because you are strong, and the word of God abides in you, and you have overcome the evil one.'
The change from present tense to aorist is probably stylistic, as he partially repeats what he has already said, although possibly emphasising more the permanence of their knowing. Once again he addresses them all first, as ‘little children' (paidia - compare John 21:5), and this time reminds them that they know the Father. In the light of chapter 1 the point is that being forgiven and knowing the Father go hand in hand. It is when we come to know the One Who is light and come to His light that we seek forgiveness, and then we come into a fuller knowledge of Him Who is light. The connection of this with the following ‘know Him Who is from the beginning' then brings together Father and Son signifying that knowing the One involves knowing the Other. They know both the Father and the Word of life, Who are both of eternity. Or the thought may be of the Father as the One Who watches over them from Heaven as His little children (John 5:45) so that they need not live anxiously (Matthew 6:8-9; Matthew 6:25-34). They are forgiven and the Father watches over them in all their needs (this especially if the little ones are to be seen as a distinct group). But the emphasis is certainly on the fact that they know what it is important to know, they know the Father and the Son and the light and the Word of life.
Because they know the Father directly (Who is the Father of every form of light - James 1:17) they have no need of spurious ‘knowledge' or intermediary ‘lights'. They are in direct contact with Him Who is the source of all things, the pure light, without shadow or lack.
Then in readiness for his coming exhortation he again refers to the fathers and young men. To the fathers he repeats what he has already said. It is all that needs to be said and emphasises that He Who is the light of the world ‘is' from the beginning, and that they know Him as such. They know Him Who ‘is' from the beginning. This is the true knowledge. To the young men he brings the reminder of the means by which they have overcome the Evil One. It is by having the word of God (or ‘the Word of God') abiding in them. For the way to overcome the Evil One is by use of the Scriptures as a weapon of defence and attack (Ephesians 6:10-17) as Jesus did during His temptations (Matthew 4:1-11), and by the indwelling of Him Who is the Word of life Who came from God (1 John 1:1; Galatians 2:20; Ephesians 3:17).
Note the connection with the opening verses of the letter. They know the Father (1 John 1:2-3), they know Him Who is from the beginning (1 John 1:1), they know the Word of God (1 John 1:1-2). Thus does he express his confidence in them and for them, and turn their eyes on what truly matters. This is necessary prior to giving them a warning concerning the world, which he later tells us lies in the Evil One, and directing them to continue to do the will of God. Right doctrine is always at the root of right behaviour.