Adam Clarke's Commentary on the Bible

Verse 8 (Romans 13:8)

Owe no man any thing, but to love one another - In the preceding verses the apostle has been showing the duty, reverence, and obedience, which all Christians, from the highest to the lowest, owe to the civil magistrate; whether he be emperor, king, proconsul, or other state officer; here he shows them their duty to each other: but this is widely different from that which they owe to the civil government: to the first they owe subjection, reverence, obedience, and tribute; to the latter they owe nothing but mutual love, and those offices which necessarily spring from it. Therefore, the apostle says, Owe no man; as if he had said: Ye owe to your fellow brethren nothing but mutual love, and this is what the law of God requires, and in this the law is fulfilled. Ye are not bound in obedience to them as to the civil magistrate; for to him ye must needs be subject, not merely for fear of punishment, but for conscience sake: but to these ye are bound by love; and by that love especially which utterly prevents you from doing any thing by which a brother may sustain any kind of injury.

- Adam Clarke's Commentary on the Bible