The big reason is that none of the epistles are stand-alone theology books. Paul, Peter, James, John, etc. are writing to specific people in specific times about specific topics. The danger is in taking a book like Romans and saying "There is Christian doctrine." It's not. And so there are some blanket statements that I read in Romans that I think should have the context of the rest of the New Testament before I consider them correct doctrine.
Concerning the specific topics, when these letters were written they were a lot of the time to correct false beliefs of the church that they were written to. For example in Romans the Jews were trying to put their Jewish laws and traditions on the new Gentile Christians. Therefore, Paul emphasizes our freedom from the law and the fact that the law is to show our sin. But I know that in James, he is dealing with people who are sluggards and are not obeying what Jesus commanded. So James emphasizes works that are brought about by the Spirit. My point is that it could be very dangerous for me to take one letter of the New Testament on it's own without the context of the others. So I'm not going to do that.
So I'm not sure how I am going to write notes on here. I may do something at the end where I write down really important things that I learned from the letters to the churches. We'll see. I'm going through them slowly and methodically so it may be awhile before I finish them. But I would encourage the people who read this to go back and read through these letters (basically Romans-Jude). Even if you have been a Christian for 40 years, there are so things in here that are really emphasized that I don't hear talked about too often. And I think it's important to understand them. But who cares what I think. God thinks it's important to understand them, otherwise why would he put them in the Bible.
That actually makes me think of a conversation I was having with my dad last night. I just got Wayne Grudem's Systematic Theology book. It's basically a big book that has the basics of nearly every doctrine discussed and explained in it. So I was thumbing through it and talking about it with my dad. I found it interesting that the first section of the book is not the doctrine of God; it is the doctrine of God's Word. As we thought about it that made a lot of sense. If God's Word is not reliable, then the rest of it really doesn't matter. If you don't believe the Scripture, then there is really no reason to read it or try to come up with a belief about God from it. The reason for that is that the Bible believes itself to be true. From Genesis through the end (I've seen it even if I haven't read all of it through) the Bible is written as the Word of God and that is what everyone accepts it as. I've already talked about how Jesus believed that the Scriptures were historically accurate by His references back to Jonah and David and Noah. He talks about them like they were real historical figures. So if they aren't real, then I don't trust Jesus because He was wrong about them.
Here's my point. There are two things that the Bible can be. 1) It can be a book that is not true. 2) It can be a book that is true. If it is not true than it has no purpose besides reading for fun and maybe for some purpose of understanding ancient literature. It's basically of no importance. But if it is true, than it can be taken as nothing less than the words of the Creator of the Universe given to us. And it is of eternal and infinite importance. So the only thing that the Bible cannot be, is kind of important. It is either the most important tangible thing that we have because it reveals who God is or it is of no importance. But it cannot be kind of important.
Just a little rant that my dad and I were having last night. I think that is very important for me to remember though. Everything that I believe stands on the legitimacy of the Bible. If that falls, everything else falls with it.
Soli Deo gloria