Piper on Holy

On I Corinthians 7:13-14

Perhaps it refers to the “sanctity” of lawful marriage and legitimate children. The marriage is still lawful (though one is an unbeliever), and the children are legitimate (though born of this mixed union). The “sanctity” (holiness) of marriage and birth are not compromised.

Corraborotion...

Then Jewett, shows from Jewish sources like the Mishna, that the Hebrew stem for holiness (kadash) is used in just this way at least ten times, signifying the setting apart of a woman to be a man’s wife. “A man ‘sanctifies,' that is, espouses, a wife by himself or by his messenger”.