The Good Shepard is one of the earliest Christian symbols in  Western art history. Throughout times past Biblically themed art-works from the Byzantine period through the High Renaissance are darkly mystical, infused with an ethereal passionate intensity and  rich Christianized symbolism.
 

The Good Shepherd symbolizes Christ the Savior "I am the Good Shepherd: the Good Shepherd giveth His life for His sheep" (John 10:11)

The Shepherd represents Jesus who will search devotedly for even one lost Child of God until it is found. (Luke 15)

The Parable of the Lost Sheep  (Luke 15)
Now the tax collectors and "sinners" were all gathering around to hear him. But the Pharisees and the teachers of the law muttered, "This man welcomes sinners and eats with them."
Then Jesus told them this parable: "Suppose one of you has a hundred sheep and loses one of them. Does he not leave the ninety-nine in the open country and go after the lost sheep until he finds it? And when he finds it, he joyfully puts it on his shoulders and goes home. Then he calls his friends and neighbors together and says, 'Rejoice with me; I have found my lost sheep.' I tell you that in the same way there will be more rejoicing in heaven over one sinner who repents than over ninety-nine righteous persons who do not need to repent.

The Parable The Shepherd and His Flock (John 10)
"I tell you the truth, the man who does not enter the sheep pen by the gate, but climbs in by some other way, is a thief and a robber. The man who enters by the gate is the shepherd of his sheep. The watchman opens the gate for him, and the sheep listen to his voice. He calls his own sheep by name and leads them out. When he has brought out all his own, he goes on ahead of them, and his sheep follow him because they know his voice. But they will never follow a stranger; in fact, they will run away from him because they do not recognize a stranger's voice." Jesus used this figure of speech, but they did not understand what he was telling them.

Therefore Jesus said again, "I tell you the truth, I am the gate for the sheep. All who ever came before me were thieves and robbers, but the sheep did not listen to them. I am the gate; whoever enters through me will be saved. He will come in and go out, and find pasture. The thief comes only to steal and kill and destroy; I have come that they may have life, and have it to the full.

"I am the good shepherd. The good shepherd lays down his life for the sheep. 12The hired hand is not the shepherd who owns the sheep. So when he sees the wolf coming, he abandons the sheep and runs away. Then the wolf attacks the flock and scatters it. The man runs away because he is a hired hand and cares nothing for the sheep.

"I am the good shepherd; I know my sheep and my sheep know me— just as the Father knows me and I know the Father—and I lay down my life for the sheep.  I have other sheep that are not of this sheep pen. I must bring them also. They too will listen to my voice, and there shall be one flock and one shepherd.  The reason my Father loves me is that I lay down my life—only to take it up again.  No one takes it from me, but I lay it down of my own accord. I have authority to lay it down and authority to take it up again. This command I received from my Father."

At these words the Jews were again divided.  Many of them said, "He is demon-possessed and raving mad. Why listen to him?"

But others said, "These are not the sayings of a man possessed by a demon.  Can a demon open the eyes of the blind?"

Symbolism in Ecclesiastical Themed Art

Most prominently featured  are  the holy symbols of the Christian faith--Christ,  Saints, The Apostles, The Cross,  Virgin Mary,  Chalice, Keys, The Anchor, Wheat ,  Animals, Fish, Angels, Birds, Insects  and Satan.