One Gospel tells you exactly why it was written.

That you may believe... and that believing you may have life in His name.

The Gospels were never meant to be a complete biography. John says so himself — if everything Jesus did were written down, the world itself could not contain the books (John 21:25). So each writer chose events that served his own purpose. Matthew presents Jesus as the King of Israel, fulfilling the promises to Abraham and David — his key verse pictures Jesus longing over Jerusalem like a hen gathering her chicks under her wings (Matthew 23:37-39). Mark presents Him as the Suffering Servant, who did not come to be served, but to serve, and to give His life a ransom for many (Mark 10:45). Luke presents Him as the Son of Man, the Savior of the world, who came to seek and to save that which was lost (Luke 19:10).

John is different. He doesn't just show you a purpose through his choice of material — he states it outright. Many other signs Jesus performed that aren't written in this book, but these have been written so that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in His name (John 20:30-31). Three things are packed into that one sentence: who Jesus is — fully God, In the beginning was the Word, and the Word was God (John 1:1) — what response He's after — belief — and what that response gets you — life in His name.

John builds his entire Gospel around that purpose — seven miraculous signs, seven "I am" statements from Jesus' own mouth, and a Gospel that opens and closes on the same note: Jesus is the Christ, the Son of God. If you want to know what it takes to receive eternal life, this is the Gospel written specifically to answer that — not Matthew's kingship, not Mark's servanthood, not Luke's rescue narrative, but John's direct evangelistic appeal.