Armstrongism affirms extra-biblical revelation through Herbert W. Armstrong's prophetic ministry. Armstrong claimed to restore long-lost biblical truths such as the seven church eras, the identity of the church of Philadelphia, and God's festival calendar. His writings, particularly Mystery of the Ages, are presented as divinely inspired and essential for understanding Scripture correctly.

While Armstrongism formally maintains the closed biblical canon, Armstrong's words are functionally treated as revelation—'God's truth restored.' The Philadelphia Church of God particularly emphasizes that Armstrong's writings are essentially inspired, even if not technically canonical. Other Armstrongist groups are slightly more cautious, but all treat Armstrong's theological and prophetic claims as authoritative divine truth. This allows for a form of continuing revelation despite formal adherence to sola scriptura.