Being called a "sheep" today carries a negative connotation, unfortunately. Despite that, Christians love to use Christ's title "The Good Shepherd". What is a shepherd if there are no sheep to follow him? The sheep are part of His flock. They may not be perfect - the wander, they get lost, and they get sick, as the first reading (Ez 34:11-12, 15-17) tells us.
We may rejoice in having a Good Shepherd, but we should tremble also. The shepherd is the master of the flock. He will separate those that do not follow Him, those that don't belong to the flock.
When Christ returns to utterly defeat the last enemy, He will come in glory and separate the sheep and the goats, the wheat and the chaff. The sheep of His flock will follow Him in victory. With that last enemy - death - defeated, those of the flock go on to eternal life.
How does he judge them? Not by whether they stumble or stray. We know the shepherd finds and corrects those sheep. He judges their worth by how they live in the flock, how they treat each other - especially the "least ones".
The lesson in today's readings, then, is that there is a victory coming, but we may stay on the victorious side. There is a Good Shepherd to accept us despite our failings, but we must accept His guidance and correction. We must retake that often-negative word and be sheep, if only because we have such a Shepherd.