In my opinion, the gospel of John is the most important book of the Bible for this reason: it is the only book in the Bible that explains that it was written for the purpose of persuading its reader that Jesus is the Christ, the Son of God, so that they may have eternal life. For this reason, I think that it is wise for someone to begin their reading of the Bible with the gospel of John, and to strive to understand that book before they understand any of the others. Do you want to live forever? Believe that Jesus is the Christ, the Son of God. Do you want to know what that means? Read the gospel of John.
Besides being a good book to understand in order to gain the one thing that is most valuable to a human being, the gospel of John also lays the foundation for the rest of the Bible. In it, we learn that God exists in three persons, the Father, the Son, the Holy Spirit, that there were eyewitnesses of the Lord Jesus Christ, among whom was the human author of the book, that the Lord Jesus Christ is God and a man, that He died for the evil deeds of people, that He came back to life, that God commands His children to love one another as Christ loved them, and how one may share the gospel with people. This does not just mean with unbelievers, but believers. The very fact that the gospel of John is called “the gospel” shows that God found it necessary to give believers the opportunity to read about the gospel of the Lord Jesus Christ over and over again.
So, have you read the gospel of John?