I believe in Christ like I believe in the sun... not because I can see it, but because by it I can see everything else.As is typical of a product of Lewis' poetic and profound mind, this line is rich with meaning. It's worth digging into a bit.
Light has always been a symbol for God. God's first line in Scripture is, "Let there be light." Opposed to this light is the darkness of chaos and evil and ignorance; light is associated with order and goodness and rationality. With light you can see what you're doing and put things in their place. With light the good is given strength, for the wicked avoid the light to hide their evil deeds (John 3:20).
And with light we see and know the world better, and thus light has long been fittingly used as a symbol for understanding. You can see this in many an ancient writer; you can see it in modern terms like "the Enlightenment." You can see it in the jazz standard "I'm Beginning to See the Light." You can see it in every cartoon where a character has an idea and has a light bulb appear over his head. Since God is the source of this order and goodness and rationality, God is associated with light.
And as God, so Christ. The Gospel of John says that Christ is "the light that gives light to every man coming into the world." Christ is the Logos. He is reason and intelligibility. He is the reason for everything that is. He is the source and the means and the goal of all understanding. All true knowledge leads us eventually to the knowledge and love of God. It is by Him that we understand anything. He is the light by which we see everything else.
Man, that Lewis was economical in his words. It took me three paragraphs to say what he said in one line. He's good. Definitely worth reading.