“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.”
If you missed it the first time around, this is where you go back and reread the book again. But really it has been rather plainly presented. Jesus is the Messiah, promised of old, sent to save the world from their sins. Trust in Him and you too can live life as God intended, in a restored relationship with God, in His Kingdom.
John hasn’t been giving us an account of Jesus’ life from cradle to grave (and resurrection). He tells us here that he has left a lot of stuff out. He chose events and moments that reinforce the theme. John is only concerned with conveying Truth, and a sampling of what Jesus did suffices. Anything else he could have chosen to tell us would have simply reinforced what is already here. Nice, but unnecessary.
So, as you might do when you get to the end of a movie or a book where a central truth is revealed in a shocking ending, you might really go back and revisit it with that idea in mind. It could change everything.