"Now to him that is of power to stablish you according to my gospel, even the preaching of Jesus Christ according to the revelation of the mystery, which was kept secret since the world began, But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith " (Ro.16:25-26).
Speaking of these verses Sir Robert Anderson writes that "I have rendered the first 'kai' in this sentence by 'even'; for it is certain that the Apostle did not mean to distinguish between the gospel of Christ and a gospel of his own!...the 'mystery' of the Body is a truth for the Christian, whereas here the Apostle's subject is the gospel which was to be 'made known to all the Gentiles' (R.V. marg.)" (Anderson, Misunderstood Texts of the New Testament [Kregel Publications, 1991], p.83).
Of course Anderson is correct. The teaching of the Church, which is His Body, is a truth which is exclusively for the Christian, a truth that cannot be understood until one is first quickened by the Holy Spirit upon believing the "gospel of grace": "But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned" (1 Co.2:14).
Paul certainly understood this and he relates that when he preached to the unsaved at Corinth he preached nothing to them except "Jesus Christ and Him crucified":
"I determined not to know any thing among you, save Jesus Christ, and him crucified" (1 Cor.2:2).
Paul also describes his message as "the preaching of the Cross":
"For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God" (1 Cor.1:18).
He says that the "preaching of the Cross" is the power of God in regard to those who are saved. This "preaching of the Cross" is the gospel:
"For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth" (Ro.1:16).
So when preaching to all unsaved Gentiles the minister is not to preach the gospel and the mystery truths concerning the Body of Christ but instead only the gospel. It would not be profitable to preach the truths of the Body of Christ to anyone before they were born again and able to understand these higher truths of the Christian revelation. Therefore it is obvious that the Christian is not to preach the mystery truths of the Body of Christ to "all nations".
It was the "gospel of grace" that was preached to all nations: "...the word of the truth of the gospel; Which is come unto you, as it is in all the world; and bringeth forth fruit, as it doth also in you, since the day ye heard of it, and knew the grace of God in truth " (Col.1:5-6).
Therefore, we can understand that it is the "gospel of grace" that is "the mystery" and it is that message that is to be preached to all the Gentiles.
"Surely you have heard about the administration (dispensation) of God's grace that was given to me for you, that is, the mystery made known to me by revelation, as I have already written briefly" (Eph.3:2-3; NIV).
Here we can see that Paul refers to "God's grace" as the mystery--"Gods grace that was given to me for you, that is, the mystery,"
Harold W. Hoehner, Professor of New Testament Studies at Dallas Theological Seminary, writes the following in regard to these verses.
"Paul was to administer 'God's grace' which was given to him...That is further explains that the mystery is the 'grace' that is mentioned in verse 2" (Walvoord & Zuck, The Bible Knowledge Commentary; New Testament [ChariotVictor Press 1983], p.628).
The "mystery" is the "grace" mentioned in verse 2. Paul also says that he has already written briefly about "the mystery" in the same epistle. Here are Paul's previous references to the "grace" of God:
"In whom we have redemption through his blood, the forgiveness of sins, according to the riches of His grace...For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast" (Eph.1:7;2:8-9).