"This mystery is that through the gospel the Gentiles are heirs together with Israel, members together of one body, and sharers together in the promise in Christ Jesus." NIV translation
Not only are Gentiles included in the relationship with God but we are joined with Israel into one body. This is only possible through Jesus.
Paul explains that it is by God's power that he is able to share the gospel. It is also only because of God's grace that he is allowed to do so.
"Although I am less than the least of all God's people, this grace was given me: to preach to the Gentiles the unsearchable riches of Christ, and to make plain to everyone the administration of this mystery, which for ages past was kept hidden in God, who created all things." NIV translation
Paul sums up his ministry here and he never strayed from God's calling on his life. He shows humility in saying that he is less than the least and knew that it was only a gift of God that allowed him to serve. His ministry was twofold in that he was to bring Gentiles to Christ and to explain to everyone how God brought Jews and Gentiles together into one body. When you serve, do you serve with the same attitude (humility and gratitude for the chance to serve)?
"His intent was that now, through the church, the manifold wisdom of God should be made known to the rulers and authorities in the heavenly realms, according to his eternal purpose which he accomplished in Christ Jesus our Lord." NIV translation
The purpose for this great mystery is for God to be glorified. Imagine the surprise when the angels and others in heaven saw Jesus crucified and then raised again. You see they don't know everything so God gets to reveal things about himself and enjoy watching His creation as they come to understand Him more. They had no idea that God was going to bring the Gentiles into a relationship with Him. Just imagine their wonder. We, too, should have that same wonder as we see what God has done for us.