I am in the beginning stages of working through John's Gospel these days, and in the process of studying chapter 1 I came across Matthew Henry's take on what Jn. 1 is all about: "The scope and design of this chapter is to confirm our faith in Christ as the eternal Son of God, and the true Messiah and Saviour of the world, that we may be brought to receive him, and rely upon him, as our Prophet, Priest, and King, and to give up ourselves to be ruled, and taught, and saved by him."
I'm not totally sure he is right about the purpose of John 1 as a whole (and he is, by the way, referring to the whole chapter, not just vv. 1-18, the "Prologue"). But it is a great quote either way, and with the addition of only a few clarifications, you could work this into a statement that pretty well sums up the purpose of the whole Bible, don't you think?