because you know that you will receive an inheritance from the Lord as your reward. It is the Lord Christ you are serving.
The apostle concludes the chapter with exhortations to relative duties, as before in the epistle to the Ephesians. The epistles which are most taken up in displaying the glory of divine grace, and magnifying the Lord Jesus, are the most particular and distinct in pressing the duties of the several relations. We must never separate the privileges and duties of the gospel religion. I.
Commenting on Colossians 3:18-25
Knowing that of the Lord ye shall receive the reward of the inheritance,.... This is said for the encouragement of Christian servants, who, though they may receive little or nothing from their earthly and carnal masters; yet they shall be used and treated as children by the Lord, and by whom they will be possessed of an eternal inheritance, after their work and labour is...
the reward of the inheritance--"Knowing that it is from the Lord (the ultimate source of reward), ye shall receive the compensation (or recompense, which will make ample amends for your having no earthly possession as slaves now) consisting of the inheritance" (a term excluding the notion of meriting it by works: it is all of grace, Rom 4:14; Gal 3:18).