Indeed materials for a full biography [of Jesus] have been withheld from men. The earliest converts were converted by a single historical fact (the Resurrection) and a single theological doctrine (the Redemption) operating on a sense of sin which they already had... The ‘Gospels’ come later and were written not to make Christians but to edify Christians already made.
CS Lewis, The Screwtape Letters (originally 1942; this edition: Harper Collins, 1996) 126.