Paul uses Colossians to express the primary theme of the Kingdom of Jesus (1:13).
Using this theme as the starting point one can see that most of the analogies and words used by Paul retain a tone of setting apart. This theme is not unlike that of Jesus and His use of The Kingdom of God (Mark 1:15)
To understand just what Paul is implying it is best if it is broken down into a proposition and then each portion of the proposition examined.
Paul claims that the gospel is the news of Jesus Christ, dead and resurrected, who delivers us from the power of sin and darkness, making us alive in today with a hope for His coming in the future.
There should be no doubt that Paul starts with Christ as the foundation stone to his gospel. Starting in Col 1:15 Paul describes the preeminence of who Christ is. This Christ, is also God (19). Christ’s blood makes peace to those who are reconciled (20). Christ’s work on the cross is used by Paul to explain how Christ delivers His people from sin and darkness (2:12). Paul uses resurrection language to describe victory (2:15). This victory theme becomes important in the discussion on how Christ delivers us.
Paul uses this ‘setting apart’ language early in the letter (1:12).
His use of “qualified you” Greek ἱκανόω contains the sense of making competent, also used in 2 Cor 3:6. This duty is to share in the inheritance of the saints in light. Light as opposed to darkness (1:13). This domain is the kingdom of Jesus. To reinforce this thought Paul uses the word “alienated” ἀπηλλοτριωμένους in Greek that is to be alienated or estranged (1:21). This alienation is reconciled by the death of Christ (1:22). This deliverance from darkness provides us the power over sin (2:13). Not only the power over sin but the complete canceling of the debt that sin brings (1:14).
Paul sees this placement into the kingdom as also a future hope.
A hope that we must set our minds on (3:2). This knowledge of the kingdom and Christ places us in the unique position to have our minds renewed (3:10). This knowledge also leads us into putting on love (3:12). Paul uses all these points to describe how one is to declare the kingdom as a herald or worker for the kingdom (4:12).
This is contrasted with how Paul expresses the gospel in other letters. In Romans Paul uses a lot of language that focuses on justification and redemption for our sins (Rom 1:16-18). The entire discourse on Abraham focuses on him being justified in faith (Rom 4). Paul’s gospel is not different in each letter but there are different aspects that Paul emphasizes. In Galatians, Paul focuses on the aspect of justified faith (Gal 2:16). As in Romans Paul contrasts the gospel with the law (Gal 3). Discourses on the law are almost not existent in Colossians. Ephesians is the most like Colossians in its gospel theme. Paul uses language in Ephesians that emphasizes the kingdom nature of the gospel (Eph 4).