(Hope you're not tired of you know who yet...)
The Apostle continues: 'But thanks be to God that you were the slaves of sin.' Is he giving thanks as one who rejoices because man was the slave of sin? Far from it. He is giving thanks not because we were previously slaves of such a cruel master, but because we are slaves no longer. He expressly makes that point clear by his next words: 'but you have now obeyed from the heart that form of doctrine into which you have been delivered, and having been set free from sin, you have become the slaves of justice.' We have become obedient, brethren, through the gift of Him who calls us, not through our own will, for we were being held as captives by it.
- St. Peter Chrysologus, Homily 114 (On Romans 6:15-21)