Wow.. a lot to dig through here so I may not cover it in one post. The first parts of the chapter deal heavily with the Apocrypha. Having grown up a Roman Catholic, I found this chapter to be quite revealing and learned some things that I didn't know.
By the way, for those who have ever wondered "What's the big difference between Catholics and Protestants?", you should go here, here, here and here.
To answer the question of "Why Does It Matter To Determine The Canon?", Grudem writes the following paragraph:
The precise determination of the extent of the canon of Scripture is therefore of the utmost importance. If we are to trust and obey God absolutely we must have a collection of words that we are certain are God's own words to us. If there are any sections of Scripture about which we have doubts whether they are God's word or not, we will not consider them to have absolute divine authority and we will not trust them as much as we would trust God himself."
Let that paragraph sink in for a while!
Grudem then traces the origins of the beginning of the Old Testament Canon to the giving of the 10 Commandments to the Israelites. Makes sense, it's the first written commands of God! (Ex 31:18) (Ex 32:16) (Deut 4:13) (Deut 10:4-5)
He then deals with a thorny issue and this is where we'll end for today. In Deut 4:2, Moses warns the people "not to add to or take away from the word that he had commanded them." But yet in the book of Joshua after Moses departs from the scene, we have that very thing occuring. (Josh 24:26). Grudem's explanation here is this:
Joshua must have been convinced that he was not taking it upon himself to add to the written words of God, but that God himself had authroized such additional writing.
Well, and here's where I'll end the systematic theology discussion for today. Isnt' that what the Mormons, Jehovah's Witnesses, and numerous other cults have claimed?