A very good discussion, from a Catholic perspective, on whether the canon is closed: Ad Simplicium Circa Scripturas at "Jimmy Akin.org". The basic point seems right to me; after all, the precise character of the canon is a canonical matter. What the community of saints has discovered is that the Spirit has worked in that community so as to establish certain books as having canonical authority. (A canon is a standard according to which other things are measured.) There is nothing to prevent other books from achieving the same authority; although it would be difficult to think of circumstances which would make it even likely. The canon we have owes itself in great manner to the fact that it grew up gradually; people looked around and saw that some books had canonical authority in the Church at large, and began to defend those books against those (like Marcion or the Gnostics) who denied that canonical authority. Even in the case of marginal books, some (like Revelation) gradually took universal hold while others (like Shepherd of Hermas) gradually faded out of canonical use everywhere.
I once had an argument with a Neo-Gnostic about the status of the Gospel of Thomas; I was denying, of course, that we could rationally treat the Gospel of Thomas as canonical. My argument was this: despite the way it is sometimes treated, the canon did not arise by people looking at texts and saying, "Yes, these texts meet these criteria." People did, of course, start formulating criteria and arguing about whether certain books met them. But this was not how the canon came about; people started forming criteria in order to hammer out apparent differences. This argument presupposed already the existence of books exhibiting canonical authority in their use by the Church. When Irenaeus talks about the Rule of Faith, for instance, it is not (contrary to some Neo-Gnostic anti-Irenaeus propaganda) in order to argue that the books he used should be canon; it is in order to argue that, unlike the books used in orthodox Churches, which are canon, Gnostic books are not even reasonable candidates for canonical status. So the criteria (e.g., apostolic origin) are not the foundation of the canon; they are theories proposed to help us think more clearly about the canon insofar as it already exists.
Now, even if we assume (as the Neo-Gnostic did) that the Gospel of Thomas is inspired by the Holy Spirit, inspiration is not sufficient for canonical status. I, for instance, believe that the letters of Ignatius of Antioch, Clement of Rome, and Polycarp are inspired. They are the works of Spirit-guided men that the Spirit has used, and will continue to use, for the good of the Church. But it is one thing to be inspired and another thing to be inspired so as to be used by the Spirit in the preaching and prayer of the Church as a canonical standard against which doctrine is to be measured. To make any judgment about whether a book is fulfilling that role, we have to see the book in action in the preaching and prayer of the Church. We barely have any indication of this in the case of the Apostolic Fathers; although if it turned out that we found ourselves needing in our doctrine and practice to rely on I Clement, it would be a reasonable question to ask whether this need is indicative of the Spirit's use of it as a canonical. As it happens, this is immensely unlikely; I Clement had its chance, and made a decent show, but did not take, and it's difficult to think of any conditions under which the canon we have would not suffice for anything that I Clement could give us. But in the case of the Gospel of Thomas, we haven't the faintest clue about the Church-context of the text -- no more than a few guesses, at least. Indeed, we don't even know if there was any canon-like function in the community that used the text; it might have just used the text as private devotion rather than public standard. Certainly, it seems to be the case that many Gnostic communities lacked anything we could clearly call a canon; it doesn't sit particularly well with most Gnosticisms to treat books as anything more than pointers to a religious experience. And so we haven't any clue how the Gospel of Thomas would have worked canonically, or how it would work canonically for us. It's just a non-issue; there is no question of whether the Gospel of Thomas is a candidate for canon. It is not impossible that some day far in the future we could look around and see that the Gospel of Thomas as preached and prayed a certain way was exercising a canonical Scriptural authority in the Church; but we need that 'preached and prayed a certain way', and that's something the Spirit sets, not any appeal to any criteria. Any criterion one might use for whether something should be canon is just something that has been proposed as a possible way to clarify our understanding of the canon; it's a guess, sometimes a reasonable guess, but only a guess, as to why the Spirit has already canonized the book through the preaching and prayer of the faithful.
See also, at the same site, the following:
* Marcion and the Canon
* What If We Found a New Letter of Paul? (Part One)
* What If We Found a New Letter of Paul? (Part Two)
As a reading of the three will show, I'm largely in agreement (I only came across these posts after I had written the above, so it's a case of unintentional convergence); even on the issue of apostolic origin, I agree that as a matter of psychological and sociological fact, a serious inclusion movement within the Church would require a text that pretty clearly was apostolic.
