To me, a slave is the property of the master, "bought for a price." There's no coincidence there, is there? As the Master's property, the slave is available to the master for whatever he needs. Similarly, a saint, in this context, is someone who has been "secured" by some authority; labled as belonging to, but not so much in a possesive, subservient way, but more of a protected way. The way someone who carries a badge has been identified as belonging to a "protected" group. "Don't mess with this guy, or you'll be in big trouble!"
As Christians, we have most certainly been bought by the blood of Jesus Himself, and therefore "owned" by God, to do his bidding. At the same time, we've been identified, marked, protected, deputized even, to free us to do the work he's called us to do. The beauty of being a "slave" to God is, he is the kindest, most loving master one could possibly imagine.
I think for me, I need to get serious about paying Him the respect he deserves as my Master, AND, to grow in my appreciation for the price He paid for me, and for the loving way He treats me. He has the right to punish me for my disrespect, and most certainly He does discipline me, but He is always watching and waiting for the chance to run to the road to meet me when He sees me coming, to welcome me as a son.