Romans 6: 15-18
15 What then? Will we sin, because we are not under the law, but under grace? In any way.
16 Do you not know that if you submit to someone as slaves to obey him, you are slaves of him whom you obey, whether from sin to death, or from obedience to justice?
17 But thank God, that although you were slaves of sin, you have obeyed from heart to that form of doctrine to which you were delivered;
18 and freed from sin, you came to be servants of justice.
(RVR1960).
the Bible declares that we are all sinners and if we are all sinners obviously we are slaves to sin because we are under the law and not under grace but there is a promise from God for humanity where God calls us out of the slavery of sin to and make us his children
When we turn to that call God receives us as a father receives his son who has returned from evil.
Some people who call themselves "Christians" say they can sin because we are under grace and God forgives us but the apostle Paul warns us that this is not right because we cannot in any way think that since we are under grace we can commit sins already that by doing so we would be becoming slaves.
We should know that if we are subject to someone we are slaves of that someone whom we please or obey.
If we submit to God we will obey his word and it will be for good but if we practice all kinds of injustice subject to sin we are slaves to our own actions and everything will be for evil.
Normally when we disobeyed God we were slaves to sin but God in his mercy called us by his grace to get us out of that slavery and make us his servants.
And we go from slaves to sin to servants of God because grace does not justify sin.
Remember that a person is a slave to what they are subjected to, for better or for worse.