John Calvin Commentary Romans 6:18

"and being made free from sin, ye became servants of righteousness." — Romans 6:18 (ASV)

And having been made free from sin, etc. The meaning is this: “It is unreasonable that anyone, after having been made free, should continue in a state of bondage, for they ought to maintain the freedom they have received. It is therefore not fitting that you should be brought again under the dominion of sin, from which you have been set free by Christ.”

This is an argument derived from the efficient cause. Another also follows, taken from the final cause: “You have been liberated from the bondage of sin so that you might pass into the kingdom of righteousness. Therefore, it is right that you should wholly turn away from sin and turn your minds wholly to righteousness, into whose service you have been transferred.”

It must be observed that no one can be a servant to righteousness unless they are first liberated by the power and kindness of God from the tyranny of sin. So Christ Himself testifies:

If the Son shall free you, you shall be free indeed (John 8:36).

What, then, are our preparations by the power of free will, since the commencement of what is good proceeds from this manumission, which the grace of God alone effects?