The purpose of the author, John, is that we might believe. Believing is only one piece of the puzzle. The Bible says that even the devils fear and tremble because they believe. In order to believe in Him, we first must know that He is real. He's not one that someone just decided to write about. John wrote many things so that we may believe. He was our eye witness. God had you on His heart before the beginning. The bible is just what God wanted, all that God wanted, and only what God wanted. The Bible has been preserved through shed blood, through wars, through acts of providence that man knows nothing about. It's been preserved and maintained through centuries. Don't trust your heart! It's influenced by your old sin nature. The more we're into the word, the less the World is into us.