James, Peter, John, and Jude were apostles to the circumcision and there is nothing in the Bible to suggest that that ever changed (Gal 2:9). That is why these letters are addressed to the Hebrews and the scattered twelve tribes abroad (Jam 1:1; 1 Pet 1:1; 2:11-12).
The books of Hebrews through Revelation are written to and about the godly remnant of Israel who will suffer great tribulation as they look for the Second Coming of Christ to the earth to establish His earthly millennial kingdom. They have both a historical application to the kingdom church in the book of Acts (a renewed offer of the kingdom by Peter) and a prophetic application for the future tribulation saints.
One needs only to stop and ask the question, “would a loving God provide specific instructions and provide prophetic insight to His chosen nation, the remnant that is committed to and follows the God of Abraham, Isaac, and Jacob in order to assist them with enduring the seven-year period that the prophets wrote about?” Indeed He has and that instruction and insight is found within the Hebrew epistles of Hebrews through Revelation. These books are not written to the Body of Christ, but are written to the multitude of tribulation saints that will be living during the time of the culmination of the prophetic kingdom program. Christ foretold of the Hebrew epistles (Jn 16:12-13). Take note of the emphasis on the “last days” of prophecy within the Hebrew Epistles: