I feel that the basic idea of a slave is that the slave is bought for a price and once bought they are to do their masters bidding. Saints, according to our Brother Paul, are those who no longer do things according to the flesh but instead are led by the Spirit of God. The gift of the Holy Spirit is given to those who have confesed Jesus Christ as their Savoiur and who believe that His death on the cross was the price He paid for me, thus I am bought. This is the common denominator for a Christian's relationship to God, that a great price was paid on our behalf and so now I willingly and obediantly do His bidding!