Slaves to ?

"All the children of men are either the servants of God, or the servants of sin; these are the two families. Now, if we would know to which of these families we belong, we must enquire to which of these masters we yield obedience. Our obeying the laws of sin will be an evidence against us that we belong to that family on which death is entailed. As, on the contrary, our obeying the laws of Christ will evidence our relation to Christ's family." - Matthew Henry

Romans 6:15-23