How do we know all of the New Testament is truly inspired? Of course the words of Christ are inspired, but what about the rest? When did God command anyone to compile the New Testament? How do we know the right books were included? I know that's a lot of questions, just something I have been thinking about lately.
There are only two good answers to this:
1. You can know that the New Testament is entirely inspired because the Church chose books that were authentic and true based on its authority and guidance by the Spirit, which is why you can trust them. However, in order to believe this, you have to accept Apostolic Succession or admit some sort of authority structure in the ancient Christian church that can govern you. So you either have to be Roman Catholic, Orthodox, or Anglican.
2. You can know that the New Testament is entirely inspired because you have read, investigated, and tested it to see if it is true and divine in all its parts. However, in order to believe this, you have to be willing to put the bible to the test. And, personally, I've never meet anyone who went this way with the result that they ended up concluding everything entirely inspired. Usually, when people go this way, they tend to discover flaws - like I did (and Luther as well) with Revelation, which I do not consider divine scripture. You have to be Protestant or non-Protestant to go this way.
There is a third answer, which is a really a non-answer, but it's popular:
3. You can know that the New Testament is entirely inspired because you believe it. In order to believe this, you can be anything. Since this belief is based on nothing, it doesn't matter what you believe. It's a position that will convince nobody who doesn't already blindly believe as you do. It doesn't have an answer for why it believes because it isn't an answer.
A fourth answer is pretty much a throw-away, but it might be the only option left for some people:
4. You can know that the New Testament is entirely inspired because god has spoken to you through all of its texts in a very personal, spiritual, non-physical, non-rational way that god has not done with anything else. In order to believe this, you can be gnostic, you can be a post-modern existentialist, you can, possibly, have had real divine experiences related to all the texts in the NT, or you can be a certified loony.