In Ephesians 3, Paul indicates that the secret (>the mystery) was revealed to him by revelation. He says that he had already written about it earlier, “briefly before.”
Some argue that Paul had no knowledge of this secret at the beginning of his apostleship and that it was only revealed to him later. Initially, like the Twelve, he would have focused on Israel with a view to the conversion of the people. Only when the Jewish people rejected the gospel would he have turned to the nations.
division
A consequence of this view is that a division is introduced into Paul’s letters. His “early letters” would then be addressed to Israel and not speak of the ecclesia, the body of Christ. Only the “late letters,” from after the Acts period, would reveal the secret that believers from the nations are one with Christ.
Ephesians 3
2 since you hear of the administration of the grace of God, which is given to me for you,
3 that by revelation the mystery has been made known to me, as I wrote briefly before,
4 by which you, when reading, can understand my insight into the mystery of Christ.
earlier
If Paul says that he wrote briefly about the secret before, according to this view, that cannot refer to earlier letters, such as Romans and Corinthians. One then interprets “before” as a reference to Ephesians 1:9. However, that is a contrived explanation, motivated by the premise that Paul could not yet have written about the secret in earlier letters.
Paul makes it clear that he already had knowledge of the secret before writing the Epistle to the Ephesians and had written about it before. His readers can understand his insight into the secret of Christ in this.
letter
If “as I wrote briefly before” were to refer to Ephesians 1:9, that remark loses its meaning. Paul would then merely be referring to something he wrote a few sentences earlier in the same letter. But he wrote a continuous letter, without chapter divisions as we know them.
Paul therefore already knew this secret; it was revealed to him at the beginning of his apostleship. That is why he also speaks about it in earlier letters, such as in Romans 16:25 and 1 Corinthians 2:7.