John Chrysostom

He desires to have them freed not only from hypocrisy, but also from slothfulness. He hath made them instead of slaves free, when they need not the superintendence of their master for the expression "heartily" means this, "with good will," not with a slavish necessity, but with freedom, and of choice. And what is the reward?

Homily on Colossians 10 ↗

Theophylact of Ohrid

He desires that they be free not only from hypocrisy, but also from carelessness. Or better, instead of slaves he makes them free, if they have no need of oversight from masters. The expression "from the soul" means to do with good will, not with slavish necessity, but freely and voluntarily.

Commentary on Colossians ↗

Thomas Aquinas

Further, they should serve willingly; and so Paul says, whatever your task, work heartily, that is, readily. And they are to do this as serving the Lord, because one who serves another because it is the right thing to do does this because of God, the source of what is right: "He who resists the authorities resists what God has appointed" (Rom 13:2); "Doing the will of God from the heart, rendering service with a good will as to the Lord and not to men" (Eph 6:6).

Commentary on Colossians ↗