1 Corinthians 7:14

sanctified--Those inseparably connected with the people of God are hallowed thereby, so that the latter may retain the connection without impairing their own sanctity (compare Ti1 4:5); nay, rather imparting to the former externally some degree of their own hallowed character, and so preparing the way for the unbeliever becoming at last sanctified inwardly by faith. by . . . by--rather, "in . . . in"; that is, in virtue of the marriage tie between them. by the husband--The oldest manuscripts read, "by the brother." It is the fact of the husband being a "brother," that is, a Christian, though the wife is not so, that sanctifies or hallows the union. else . . . children unclean--that is, beyond the hallowed pale of God's people: in contrast to "holy," that is, all that is within the consecrated limits [CONYBEARE and HOWSON]. The phraseology accords with that of the Jews, who regarded the heathen as "unclean," and all of the elect nation as "holy," that is, partakers of the holy covenant. Children were included in the covenant, as God made it not only with Abraham, but with his "seed after" him (Gen 17:7). So the faith of one Christian parent gives to the children a near relationship to the Church, just as if both parents were Christians (compare Rom 11:16). Timothy, the bearer of this Epistle, is an instance in point (Act 16:1).

— Jamieson-Fausset-Brown Commentary (public domain)