The mystery concerning the gospel is revelation that didn’t exist in objectified (written down) form until God revealed it to a man in the first century. This revelation is not according to any Old Testament scriptures nor Matt. Mark or Luke … which re technically the Old Testament also. This revealed mystery contains the sandals of peace or ‘feet shod with the preparation of the Gospel of peace” which Christ taught us by implication of teaching it to Paul in Ephesians 6:15-18.
This gospel is not to unbelievers to believe in to be saved. This gospel is only to maturing Saints in Christ … the only people who can have the faith to obey it. Carnal believers cannot comprehend this mystery revealed in the word of God because they don’t have the illumination from God the Holy Spirit required to comprehend such a wonderful truth.
Romans 16:25-27 is how we know its not to unbelievers. This is the gospel Paul was discussing in Galatians 1 also. He asked the Ephesian saints to pray concerning him that he boldly proclaim this mystery gospel … and I believe its the gospel he was referring to in saying ‘woe is me if I preach not the gospel’.