Many people believe that you can't know for certain that you know God and have eternal life. They believe you can only live the best that you can and hope that God accepts you. John blows that belief out of the water when he says that he wrote so that we could know. The key to life is in His Son. Whoever has the Son has life and whoever does not have the Son does not have life. John wrote so that those who believe in the Son of God may know that they have eternal life.