The word mystery means something true that has not been previously revealed by the Prophets of the Old Testament. The church as it is both Jew and Gentile was never revealed in the Old Testament. Another mystery was the promise of the Messiah (yet unnamed) who is found in the Old Testament's prophetic writings from Genesis to Malachi which pointed to the Jesus of the New Testament.
Paul reveals a truth about Christians of all kinds, that if we have something of quality preaching and teaching etc, we got it as a gift from God and not from our own natural wisdom. To believe otherwise would be pride. Paul is sarcastic to the Corinthians when he says: "We are fools for Christ but you are wise." (vs. 10) They were trusting in their own thinking which was completely opposite of God's wisdom. Paul speaks of Apostles as being despised people which was true of himself, who ended up later as a prisoner in Rome and his life ending there in complete disrepute.