I thought this was simply the writings of the Apostle Paul, and as simple as the content of his epistles which is new compared to the Old Testament. Reading H. Lavern Schaffer’s section on ‘the mystery of The Faith’ (Maturing in Christ pages 264-265) today has me certainly rethinking that however now. He said it was already known before the revelation given to Paul (in a nutshell).
The Faith is the body of doctrine contained in the New Testament for all Christians to know to and how to overcome their 3 spiritual enemies. ‘The mystery of The Faith’ is something to hold fast to for a deacon in a local church who has charge over the funds of the Church and distributes them according to “The Faith”. The Deacon who holds fast to that will not arbitrarily give money away to random causes.
The Mystery of The Faith is a little more involved than that which can be understood at a cursory glance.