"To them God has chosen to make known among the Gentiles the glorious riches of this mystery, which is Christ in you, the hope of glory." NIV translation
Paul sums up the mystery with the fact that we, as believers, are the temple of God. God lives in us through the Holy Spirit and that was only made possible after the temple was cleansed (our sins removed).
"We proclaim him, admonishing and teaching everyone with all wisdom, so that we may present everyone perfect in Christ." NIV translation
Paul tells us, now, why he and his team do what they do and the simple reason is that they want us all to glorify God with our lives.
That is why they (and we) warn people that do not know Jesus of the coming judgment and teach those that know Jesus how to live for Him. (That is also why we do these online Bible studies.)
"To this end I labor, struggling with all his energy, which so powerfully works in me." NIV translation
Paul reminds us that not only is God in control of all things but He is also the sustainer. It is only through God's energy and power that Paul could serve and the same is true for us. If you try to do things (even good things) on your own, you will grow weary but, if it is God's will and plan for you, He will give you the energy and power to do them.