Vanishing Beggar's Gold 19

The children were slaves. Their parents had been deceived into selling their children to masters who promised to teach them a trade and educate them. No such intention was in the minds of their new masters.
Through international pressure, an extensive rescue operation was mobilized, resulting in the actual rescue of many enslaved children.
When Paul begins writing chapter 12 of Romans, he has an actual rescue in mind. Beginning in chapter 1 he described the misery of slavery to sin. By chapter 11, he had finished describing the whole spectrum of God's wonderful rescue operation and how it actually comes to suffering, ruined human beings.
Because those children were actually rescued from slavery, they could live a very different lifestyle; one of freedom rather than slavery. Because God's people have actually been rescued from slavery, they too can live a very different lifestyle; that of freedom rather than that of slavery.

Paul here has the mercy of God in view; not in a narrow sense, but is a broad, sweeping sense. Consider the wonders of God's outstanding mercy in His great rescue of de-meriting sinners.