Paul begins Eph 3 by saying that he is a prisoner "for the sake of you Gentiles."  His proclamation of the Gospel to the Gentiles sparked a riot in Jerusalem that led to his arrest and eventual transfer to Rome.  All this is detailed in the Book of Acts.  He mentions this by way of describing further "the mystery of Christ" (Eph 3:4).

        

The difficulty Paul faced with the Jews and also with the Jewish believers in Jerusalem was finding precedent in sacred tradition for the revolutionary Gospel he was proclaiming.  He admits quite freely that he got it "by revelation" (Eph 3:3).  In other words, it was a "Moses and the burning bush" moment.  This is just the kind of thing that subsequent believers are warned against:  "don't trust your own experience, line it up with Scripture, check it out with mature believers."   I guess different rules apply if you're an apostle.  But because it was a revelation, it was not something that anyone else could have figured out, or deduced from prior revelation:

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           you will be able to understand my insight into the mystery of Christ,  which was not made known to men in other generations as it has now been revealed by the Spirit to God's holy apostles and prophets (Eph 3:4-5).

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Now this gets very dicey.  We have seen all the agony Israel went through at the establishment of the Law (Exodus through Deuteronomy):  they did not willingly or easily bend their necks to this yoke.  But once accepted, it became fundamental to their personal and corporate identity.  To be a Jew was to be one who knew and kept God's Law.  And that Law was permanent and eternal, a part of the fixed order of creation.  And then here comes Paul saying, "Excuse me, the rules have changed.  That old Law doesn't apply anymore, in fact the opposite is now true.  Red is yellow and yellow is green.  How do I know?  God told me so."  To a traditionalist, this is how it came across -- wackiness,  apostasy, blasphemy.  It was the outright denial of everything Judaism stood for.

         

Paul did not expect to convince them -- the Jews were now outside the circle of revelation, they were "in the flesh," they were out of touch with the Holy Spirit who gave revelational truth.  Since they did not accept the truth about the Messiah, there was no chance they would receive the truth about the Gentiles, for the latter was dependent on the former.  Paul's greater problem was the Jewish believers, those who worshipped Jesus as the Messiah, but did not consequently open their arms to the Gentiles.   This is why he writes of the revelation of the Spirit to the apostles and prophets,  both plural (Eph 3:5).   "It's not just me," he is saying, "God said the same thing to the other leaders."  The examples we know of from the Book of Acts are God speaking to Peter through a vision before the conversion of Cornelius, and Philip reading Isaiah with the Ethiopian eunuch.  So Paul is right that God did reveal the mystery to multiple apostles.  But the experience of God's revelation by different leaders did not mean that their interpretation of His purpose was identical, as we know from the continuing disagreements among Peter, Paul and Barnabas.  Paul was easily the most radical of the group, in terms of the obsolescence of the Law and the end of the division between Jew and Gentile.  

         

One of the signs of the continued tensions within the early church over the matter of the Gentiles is the bifurcation that occurred, not between Jew and Gentile per se, but between Peter and Paul, where one was officially given the apostolate to the Jews and the other to the Gentiles (Gal 2:7-8).  This shows that the old hatreds were still alive.  Otherwise, why did they not divide geographically?  Paul could have been given oversight of all congregations in the Greek cities, while Peter oversaw Israel and Egypt.  Instead we have Paul angrily leaving the Jewish synagogue in Antioch and from then on preaching only to the Gentiles (Acts 13:46). There was a great risk of two different versions of Christianity being promoted, one for Jewish and one for Gentile believers.