[af_vet_1981]

How was Hebrews likely was originally a synagogue address?

• It was an epistle to Jews who already believed in Jesus as Messiah.
• The Apostle Paul signed his letters which were written to Jews and Gentiles in the churches of Greece, Asia, and Rome.
• It may have been written to one, more, or all of the churches largely comprised of Jews (not synagogues, they were already called churches) throughout all Judaea and Galilee and Samaria, but need not have come from Paul.
• Those addressed in the epistle were Jews who already believed in Jesus. Paul made many appeals to Jews in the synagogues of the Diaspora, and founded churches with those who believed. The epistle does not seem intended for that audience, nor from an evangelistic appeal to a synagogue audience.
• In short, the precise identity of the author and the audience are shrouded in ancient mystery.
  
  

[daniel1212]

It was an epistle to Jews who already believed in Jesus as Messiah. The Apostle Paul signed his letters which were written to Jews and Gentiles in the churches of Greece, Asia, and Rome. It may have been written to one, more, or all of the churches largely comprised of Jews (not synagogues, they were already called churches) throughout all Judaea and Galilee and Samaria, but need not have come from Paul. Those addressed in the epistle were Jews who already believed in Jesus. Paul made many appeals to Jews in the synagogues of the Diaspora, and founded churches with those who believed. The epistle does not seem intended for that audience, nor from an evangelistic appeal to a synagogue audience. In short, the precise identity of the author and the audience are shrouded in ancient mystery.

I myself have argued against Paul being the author of Hebrews on the basis of the contrasting style and absence of his otherwise uniform statement of his authorship, yet 2 Peter 3:15 seems to refer to this epistle, and thus I postulate that it may have been originally a synagogue address (to Christians meeting therein) that was penned by someone else, as with the gospels and the teachings of Christ.

But if you want to contend against the Pauline authorship of Hebrews than that is fine with me. However, then you need to thus attribute error to your own council of Trent:

the books that are received by this Synod. They are as set down here below...fourteen epistles of Paul the apostle, (one) to the Romans, two to the Corinthians, (one) to the Galatians, to the Ephesians, to the Philippians, to the Colossians, two to the Thessalonians, two to Timothy, (one) to Titus, to Philemon, to the Hebrews (The Council of Trent - Session 4; http://www.thecounciloftrent.com/ch4.htm)