This is truly a pregnant metaphor.
If I were to apply hosepipe's insights at 4353 to the situation at hand, the pen the sheep in the pasture might go back into could be a pen of his own imagining, for good or ill.
But Jesus Christ is the Gate and the Good Shepherd. So the sheep is actually free of the pen to pasture no matter what he thinks.
Sanctification is, after all, a walk.
A pen of imaginings is a pen as real as any cage.. Actually the metaphor pen is just that, I think.. A pen of imaginings.. What is a denomination but an imagined corporation.. with a charter.. i.e. door knocks, hand shakes, and hoops to go through.. For that matter the Moose, Elks, VFW, and Boy Scouts are denominations..
There is no end to groupie variations.. Which makes the word "eklesia" (called out ones) even more pertinent..
Indeed the pen is within you.. Some are penned way up.. Jesus did not forbid pens.. but he did not seem to be real happy with synagogue's.. Which was a synthetic temple.. since they lost the ark of the covenent.. No Ark; and the temple was meaningless.. The temple became where God used to be.. I guess you must keep up with the shepherd and the flock or you get yourself "LOST".. With No shepherd a pen might seem protective.. even though its a spiritual health risk.. With all that ammonia and stuff in your hooves..
The Lord is my shepherd, I shall not want, He maketh me to lie down in green pastures
No pens, mental or otherwise for His.