Throughout this letter, John makes it very clear as to exactly why he's writing. All of his reasons tie together, focusing at their core on the foundation of our relationship with the Lord. John is offering encouragement and security to the believer, reiterating the truth's of God that, as he says several times, “we've known from the beginning.” And this is just another undeniable facet of that. If we are in Christ, we shouldn't be sinning, because we are new creations with redeemed souls. Our make-up is entirely different now than before we knew Him, and there is no room for sin anymore. Yet we still live in a fallen world and are affected by it as residents here. We still find ourselves falling back into the old ways of sin.
John writes to us, he says, “so that you don't sin.” Well, how is he going to keep us from sinning? By keeping us focused on what matters. For this is when we sin – when our hearts and minds are set on things aside from eternity. John writes to us truth and truth abundant, reminding us of who God is and what His Son did (and is doing) for us. He reminds us of this world's limits and the Father's infinity; and he instructs us on how to love one another, to stand firm, and to discern Christ in our fellow man.
Our mindset, for it is the cry of our immortal souls, should be on the Father and His presence in our lives. It is a living relationship, real and vibrant, and indicative to our existence. Without Him, we are nothing; and so without Him as our focus, we dwindle into nothing. Therefore, John gets us on track, gets our sites set on the Word of Life so that we don't stumble into death (that is, sin).
But we do sin. And what happens then? Ah, the glorious security of Christ! For John assures us here, though it is no reason to sin, that we have this advocate. When we stand before the Father upon that fateful Judgement Day, if we have cast our burdens upon His Son and given our lives to Him, we will be found redeemed in the eyes of eternity. Praise the Lord for His wonderful mercy on us, His Church!