I totally disagree with the first response above... IMHO, Christ is the founder of 'christianity'... I follow NO man... just Christ my creator, lord, and saviour.
There is SOOOOO much more to the Bible (The WORD) than those 'letters' written by Peter and or Paul.
To the OP.
The bible is a collection of 'writings', but not 'letters' (as in we write a 'letter' to someone). Could God, with his own hand/powers, have chosen to write all of this and then left it in the form of some sacred scrolls or something??? Yes, quite definitely he could have. But, in consideration of the fact that Christianity is a living and progressing spiritual religion... with a definite history from Adam and Eve, to the Jews, to the coming of Jesus Christ, it would only be natural that this story has been been unfolding, and has been written (as inspired by God) over many years....
The fact that it is not a single document, penned by one man, is actually testament to the fact that it is inspired by God... Study of the Bible shows that things written by men inspired centuries earlier actually prove and validate and work together with what is written by inspired men centuries later in a way that can be explained as divine providence.
Except for these scriptures that are considered to be the inspired word of God, the Bible tells me that no man shoud add one single letter... The Bible says that one follows the 'words of man' in vain... But, that truth, life and salvation are found in the Word of God.