"But now he has reconciled you by Christ's physical body through death to present you holy in his sight, without blemish and free from accusation -" NIV translation
Our evil behavior (sin) had to be paid for so that we could have a renewal of our relationship with our Creator. The payment for sin is physical and spiritual death and Jesus suffered through both on our behalf. His blood covered our sins just as the blood of the lamb covered the houses of the Israelites and kept their firstborn sons from being killed (see Exodus 12). Jesus was the perfect sacrifice and, when He took on our sin, he left us without blemish and justified (free of accusation).
"if you continue in your faith, established and firm, not moved from the hope held out in the gospel. This is the gospel that you heard and that has been proclaimed to every creature under heaven, and of which I, Paul, have become a servant." NIV translation
Paul reminds the Colossians that the gospel is all about Jesus Christ who died for our sins and rose again from the grave.
In the face of this teaching about a special secret knowledge, Paul just says we need to keep focused on the truth of that. That is what they started out believing and he urges them to hold on to the truth.
"Now I rejoice in what was suffered for you, and I fill up in my flesh what is still lacking in regard to Christ's afflictions, for the sake of his body, which is the church." NIV translation
How could Paul rejoice in all that he went through such as beatings and prison? Here, he tells us that he knew that it was for the good of the body of Christ and he was willing even to the point of death. If we stay focused on telling others about Jesus, we can rejoice even when it is difficult and He will give us the strength to endure.
"I have become its servant by the commission God gave me to present to you the word of God in its fullness - the mystery that has been kept hidden for ages and generations, but is now disclosed to the saints." NIV translation
Paul reminds them that it was God who sent him to share the gospel with the world and therefore it is God who is in charge. He also tells them that he has not held anything back but has presented the full gospel to them (no secret knowledge was necessary).