I write to you, fathers, because you know him who is from the beginning. I write to you, young men, because you have overcome the evil one. I write to you, little children, because you know the Father.

 His object in writing is that they may not sin. There is a two-fold connection of thought here--with what has gone before and with what comes afterwards. There is a two-fold danger that they may indeed think lightly of sin. 