Object of Gospel of John

But these are written, 

that ye might believe that Jesus is the Christ, the Son of God; 

and that believing ye might have life through his name.

John 20:31

"The object of writing the Gospel is twofold: 

1- to furnish ground for confidence that there was the incarnation of the divine nature in the person of Christ  

2- and that this incarnation has for its object to give eternal life to mankind."

W.W. Prescott