Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.
Not every biblical author so clearly states his purpose, but John does so in his gospel. He wants his readers to be firmly convinced that Jesus is the only way to life. And, if you read back through John’s gospel, you will find that this purpose undergirds everything he writes.
whoever believes in him may have eternal life.
“For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life.
Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God.
Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him.
Truly, truly, I say to you, whoever hears my word and believes him who sent me has eternal life. He does not come into judgment, but has passed from death to life.
Jesus said to them, “I am the bread of life; whoever comes to me shall not hunger, and whoever believes in me shall never thirst.
For this is the will of my Father, that everyone who looks on the Son and believes in him should have eternal life, and I will raise him up on the last day.”
Truly, truly, I say to you, whoever believes has eternal life.
Jesus said to her, “I am the resurrection and the life. Whoever believes in me, though he die, yet shall he live,
I have come into the world as light, so that whoever believes in me may not remain in darkness.
This is by no means exhaustive from the Gospel of John, but it does make one thing clear, God’s purpose in this book is the same as John’s. He wants us to see Jesus, in all His glory, and believe in Him, so that we might have life.