The canon of the New Testament was created by a process of coming to consensus among the bishops in the churches.  This process began in the late first century and it was nearly complete by the end of the second century.  Naturally, certain books were accepted by all as canonical earlier than others.  This would include the four gospels and most of the letters of Paul, which were already accepted by all as inspired by the end of the first century. They could not all have been accepted as canonical at the same time!  The Muratorian fragment and other documents from the second half of the second century, as well as quotes by early church fathers testify that all twenty-seven books now accepted by consensus of all believers were already either fully accepted or at least thought to be canonical by the great majority by the second half of the second century.  By the early third century there was virtually no debate over the list of books.

The books which were completely accepted by consensus later were 2 Peter, 2,3 John, Hebrews and Revelation.  For Hebrews, it was because the author was not known for sure.  For Revelation, it may have been because it is so different from the other inspired books.  Also, the heretical group known as the Montanists (mid to late second century) were so into using this book, that this may have caused some to hesitate to fully accept it.  For 2 Peter, not all were convinced it was written by Peter.  The same is true about 2,3 John having been written by John.  However, like I said, by the end of the second century, consensus was already forming around all of these books.

I found one reference which claims that certain Nestorian groups do not accept this book.  This must be the “Syrian Orthodox Church” you are referring to, but it is very dubious.  The fact is that there is a Syrian Orthodox Church and it has accepted Revelation from the very beginning of that group as a separate identity.  I do not know where you found this claim, but I can say most likely it is coming from someone with an agenda, trying to create the impression that the canon is not as clear as it is.

Remember, almost by definition, some book had to be accepted by consensus after others.  The fact that Hebrews or Revelation were fully accepted a generation or two later than Matthew or Romans is not evidence that it is not inspired.  I will admit that, in the end, I accept the authority of all twenty-seven books by faith in God acting to give us a scripture which is inspired, but history strongly supports that the books we have were accepted fairly early and no other books were given strong consideration for inclusion.  We can be very confident of the New Testament canon.

Some groups, such as the Ethiopian/Coptic Church and the Roman  Catholic Church accept additional books to the Old Testament, but no major Christian group today has any other list than the twenty-seven accepted by all.  The canon of the New Testament is not in doubt at all.

John Oakes