15What then? Should we sin because we are not under law but under grace? By no means! 16Do you not know that if you present yourselves to anyone as obedient slaves, you are slaves of the one whom you obey, either of sin, which leads to death, or of obedience, which leads to righteousness? 17But thanks be to God that you, having once been slaves of sin, have become obedient from the heart to the form of teaching to which you were entrusted, 18and that you, having been set free from sin, have become slaves of righteousness. 19I am speaking in human terms because of your natural limitations. For just as you once presented your members as slaves to impurity and to greater and greater iniquity, so now present your members as slaves to righteousness for sanctification. (Romans 6:15-19, NRSV)
When we follow after something or someone and submit to them then we are a slave to them.
Christ has set us free to live the life God has for us. And whenever we follow another we are leaving freedom behind and becoming a slave to another.
So we must be a slave of God and follow after his plan for our lives and make ourselves slaves to righteousness for sanctification!