Those who employ metaphors to convey their ideas normally only zero in on one part of the comparison, ignoring other parts that don't fit the idea they're trying to get across. For instance, when we call someone "honey," we're usually referring only to the sweetness of that substance, ignoring the fact that bees produce honey through a process of regurgitation. Or when the Song of Songs author compares his lover's nose to "the tower of Lebanon that looks toward Damascus," he's not referring to the size of that tower.
In the same way, the shepherding metaphors employed in today's second and third readings zero in on the care shepherds and gate keepers have for their flocks, not on the stupidity of sheep.
In a highly institutionalized church, which Catholicism has become, it's at times hard for us to cut through the minutiae of the structure and return to the basic relationship with the risen Jesus which was at the heart of early Christian faith, a faith which was there long before the development of the institution.
Already by the end of the first century, some in John's community were breaking their concentration on Jesus and his care of them. They were starting to integrate elements into their beliefs which had nothing to do with imitating Jesus' dying and rising. They had forgotten, in the words of Peter in our Acts pericope, that they had been "baptized ... in the name of Jesus the Christ for the forgiveness of... sins." Once Christians lose sight of their being other Christs, strange things begin to happen.
That's why John insists on a return to Christian basics. Like an enclosure provides security for sheep, so our relationship with the risen Jesus provides security for us. He's the only gate we'll ever need. "I am the gate," John's Jesus insists. "Whoever enters through me will be saved, and will come in and go out and find pasture. A thief comes only to steal and slaughter and destroy; I came so that they might have life and have it more abundantly."
In some sense, when the Pentecost crowd asks Peter, "What are we to do ...?" they're implicitly asking what they have to do to have a more abundant life right here and now. They're not necessarily inquiring about what they have to do to get into heaven.
As a child I was guaranteed entrance into heaven by just following the commands of the institution. Couldn't argue with that. Yet, though I hate to admit it, I often envied some of my Protestant friends who, even if they might not be doing what was necessary to get into heaven, were having a lot more fun than I was. They certainly were living life more abundantly. Sometimes, in contrast, I felt like I was just a dumb sheep forced to do what the smarter shepherds insisted I do for my own good. In those days, the only other Christs in my life were those shepherds.
We had forgotten what the unknown author of I Peter took for granted, that all Christians are expected to "follow in his (Jesus') footsteps." What he did, we now do. Yet the writer realistically presumes that straying from a relationship with Jesus is an occupational hazard for followers of Jesus. That's why he reminds his readers, "You had gone astray like sheep, but you have now returned to the shepherd and guardian of your souls."
That's why we, like our sacred author, always must return to the essentials. They alone guaranteed we'll live fulfilled, abundant lives long before we step into eternity.