As the oldest living apostle and chief leader among the churches, John had the right to address the believers as his "little children." He is writing to ensure they don't fall into sin by disbelieving in Christ. The intention of the antichrist is to get them to doubt that Jesus was the Christ - to move them to unbelief! John assures them that this sin of unbelief can be conquered through the power of the Holy Spirit.
Jesus not only paid the price for the sins of the those who believe, but for the sins of the whole world. We have an advocate as a friend, a Comforter who has access to the Father, who by the Spirit, abides with us, showing that we have not lost ground but are His - and this Comforter confirms this to us.