In Paul's time it sometimes occured that a slave might become the adopted child of their former owner. At this point, not only would they become a free person, they would also now enjoy the privielges of being a member of the family. It is this type of scenario that Paul is using to compare with how we were once slaves to death, but now we are part of a family of life.

In modern times, adoption is a means by which children who are not being cared for by their families of origin, to become part of a new family in which they can thrive and be safe.

Note: This is the ideal. Unfortunately in our fallen world this is not always the lived experience of adoptees