Colossians 1:23

"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.

Colossians 1:24

"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.

Colossians 1:25 & 26

"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).