2/221 — The measure is not membership but direction
"…A believing bondwoman is better than a mushrik woman, though she please you… They call to the fire; God calls to the garden and to forgiveness by His permission."
The ground of the verse is given plainly at the end, and the ground is about the direction of the call: ulāʾika yadʿūna ila'l-nār.
The point to be recorded: the verse issues no wholesale ruling about a lineage, a people or a community; it speaks by way of an attribute. At verse 62 of the sūra it was treated that what one receives is tied not to the name of a community but to an attribute; the measure here comes from the same place.
And in making the comparison, social standing is expressly set aside: wa law aʿjabatkum — "though she please you". Given the distance between slave and free in the society of that day, what the sentence does becomes visible: the measure is taken from status and attached to direction.