The Greek term from which "mystery" was translated, within the context of Paul's words and meaning, is "hidden wisdom." Where that wisdom was hidden is of the greatest importance. Given that this particular mystery was hidden in God from the creation of the world, and given that we are told WHY it remained hidden in God, in that the demons would never have sought the crucifixion of Christ had they known the mystery, the impact of this wisdom makes a huge difference for us all.
In other words, if Paul's Gospel were the same as that of John the Baptist, and of Christ Jesus, and of the eleven, then it was not a mystery revealed to Paul since Paul knew what they had been preaching. So, if it had not been a mystery, then that would make Paul a liar, and Israel being cut off, and therefore unable to be the means through which Gentiles found salvation, the entire world would be lost had it not been GRACE through faith that became the new means for salvation for both Jews and Gentiles.
So when someone demands that works are still necessary as something they think that they can use as an additive to grace, they are fooling only themselves. If they think that their tithes, water baptism, works through sacraments and religious vigor, et al, can in any way add to the GIFT of salvation through faith, it's ludicrous. They know not the scriptures.
Therein is why such things as "tradition" had to be thrown into the mix by those religions out there because they know that scripture doesn't back their dogmas. They know that historic and modern departures from the word of God can only stand in the hearts of men by supplementing and even replacing the word of God with the inferior teachings of men. Get men to doubt their salvation on the various basis of man-made teachings out there, and they have a following like no other. Jesus illustrated this nicely:
John 5:43 I am come in my Father's name, and ye receive me not: if another shall come in his own name, him ye will receive.
How many men have there been throughout history who come in their own name or title who have enjoyed almost absolute reception and accolade from the world of religion and even the secular world? Many. One of them is today teaching that all religions have the same access to the path to God and salvation without having to change their beliefs. That is ecumenicalism gone wildly rampant!
MM