And in a very interesting way we do know from history that when the early church came to define and to determine the Canon of the New Testament - there were large numbers of Christian writings by then, and the question was what should be put in and what should be left out - we do know that the Holy Spirit led the early church to decide in this way: they said that unless a document purported to be a Gospel or an Epistle and could be traced back to an apostle, either directly or indirectly with apostolic authority, it should not be included. The test of apostolicity was the text that was employed by the early church in the wisdom given to it by the Holy Spirit in determining the New Testament Canon. Now all this is indicative of the fact that an apostle is a man with unique authority; he is given the doctrine; he is given the truth. The Lord gives it to him; the Holy Spirit guides him, and he transmits it. He is a chosen servant, specifically sent to represent and to speak for the Lord in this way.
So - if you think Paul didn't write Hebrews, what evidence do you have of an apostle behind it? And if the answer is 'None', then why is it in your Bible?