Jamieson, Fausset and Brown on 1 Corinthians 7:17

17. But--Greek, "If not." "Only." Caution that believers should not make this direction (1Co 7:16; as Alford explains it) a ground for separating "of themselves" (1Co 7:12-14). Or, But if there be no hope of gaining over the unbeliever, still let the general principle be maintained, "As the Lord hath allotted to each, as God hath called each, so let him walk" (so the Greek in the oldest reading); let him walk in the path allotted to him and wherein he was called. The heavenly calling does not set aside our earthly callings.

so ordain I in all churches--Ye also therefore should obey.