At the end of John we have these words: “Jesus did many other miraculous signs in the presence of his disciples, which are not recorded in this book. But these are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.
Most often, John’s intent (writing that the reader might “believe”) is mistakenly taken to mean that it was written to bring the unsaved to Christ. But no one, unacquainted with Christianity, would be likely to come to Christ on the basis of the Gospel of John. In fact, I am sure they would be fairly confused by it.
John begins by talking about “The Word” and goes on to say that the “Word” is “God,” a fairly nonsensical statement in the ancient world unless you know and accept that Jesus is in fact that “Word.” As the story opens, the reader is expected to know who John the Baptist is (without even calling him “the Baptist”). Jesus appears without warning or identification except for John the Baptist calling him “the Lamb of God who takes away the sin of the world.” You have no idea where Jesus came from. The reader is expected to understand that the story takes place in Palestine, but not expected to know Jewish customs nor much about Palestinian geography. In other words, the reader of John is expected to already have some knowledge of the Jesus story, enough to have already become a Christian. (Note, for example, that John tells you Mary, the sister of Lazarus, was the one who had poured perfume on the Lord and wiped his feet with her hair – 11:2. John assumes his readers already know the story, because he doesn’t tell it himself until the next chapter.)
But why then does John write to encourage “belief” among those who already “believe”? The answer lies in the meaning of the word “belief.” Its basic meaning is to “trust,” and that’s something God’s people have always struggled with. It’s one thing for me to mentally agree that Jesus is the Son of God. It is entirely another for me to trust Jesus. It requires no response from me to agree to Jesus’ identity. But trusting Jesus requires action demonstrating that trust. John writes to convince his readers to do the latter.