Slave- A slave for God to do God's work,of one's own free will, relying on the Grace of God and not their own strength that they may serve. Not as a slave that was a victim of tyranny, of a cruel master - rather a slave who had the option to be free of the master but made a choice, to be bonded to the master for life. You need to give up everything for the Master - now that is committment - Do I have it??? Time to pray again - Praise Him. I am a slave that has the only means of showing their love for the Master by making a committment to serve their master for the rest of thier life. A slave whose only motive in life is to please their master, not to serve what is pleasing to themselves.
Saint - the keyword is ownership - we are reffered to as saints because of the God's (jealous) claim on us and his overwheliming love for us presented by his Son Jesus Christ. If you are owned, you accept a role essentially as a slave; in a committement described above in "slave".