For - proof that “faith” (Heb 11:13) was their actuating principle.
declare plainly - make it plainly evident.
seek - Greek, “seek after”; implying the direction towards which their desires ever tend.
a country - rather as Greek, “a fatherland.” In confessing themselves strangers here, they evidently imply that they regard not this as their home or fatherland, but seek after another and a better.