Being a slave means committed to someone for lifetime unconditionally and that master owns a slave.
saint means being dedicated to the service of God.
Although the difference between 'slave ' and 'saint' in socioeconomic context is vast but as a christian, in a spiritual context both mean the same.
my life is a gift of God to me and when i dedicate my life to God and in His service, i will definately accept His will in my life. When i allow God to decide for me that means i commit myself unconditionally. This is the essence of being a slave and saint in christian sense - an unconditional committment to God's service.