1 Verily, verily, I say unto you, He that entereth not by the door into the sheepfold, but climbeth up some other way, the same is a thief and a robber.

From verse 6 we learn that this is a parable, a representation of heavenly things through the medium of earthly things.
Christ, says Calmet, having declared himself to be the light of the world which should blind some while it illuminated others, John 9:39-41, continues his discourse, and, under the similitude of a shepherd and his flock, shows that he was about to form his Church and that into it he would admit none but those who heard his voice.

[Verily, verily] Our Lord introduces this discourse in a most solemn manner, Verily, verily! -- Amen, Amen! -- It is true, it is true! -- a Hebraism for, This is a most important and interesting truth.
Our Lord always speaks the truth, but when he delivers truth with this particular introduction it is to call the minds of his hearers to special attention.

[He that entereth not by the door] Sheepfolds were walled areas where flocks were gathered for a time, for instance, overnight, so that the shepherds could sleep. The entrance to the sheepfold was a narrow opening which sometimes had a permanent door, or, in more temporary situations, in which a shepherd could lie or sit and serve as the "door."
Christ assures us in verse 7 that he that he is the door; whoever should attempt to enter the sheepfold by other than the door does not have good intentions.

2 But he that entereth in by the door is the shepherd of the sheep.
3 To him the porter openeth; and the sheep hear his voice: and he calleth his own sheep by name, and leadeth them out.

[To him the porter openeth] He who is authorized to be caretaker of the sheep need not attempt entry by some other means.

[The sheep hear his voice] Though there may be many flocks resting within the sheepfold, each flock recognizes its own shepherd and responds to his call.

4 And when he putteth forth his own sheep, he goeth before them, and the sheep follow him: for they know his voice.

[He goeth before them] Sheep naturally follow the shepherd from pasture to pasture. The shepherd is responsible for their care and safety.

[I am come that they might have life] My doctrine leads to life because it is the true doctrine - that of the false and bad shepherds leads to death, because it neither comes from, nor can it lead to that God, who is the fountain of life.

11 I am the good shepherd: the good shepherd giveth his life for the sheep.

[I am the good shepherd] In verses 7 and 9 Jesus described himself as the door, the entrance into eternal life, now he changes the thought and calls himself the shepherd because of what he soon will do for them that believe in him, in order to prepare them for eternal glory.

[Giveth his life for the sheep] Not risks his life, Jesus, the Good Shepherd, gives his life for the sheep.

12 But he that is an hireling, and not the shepherd, whose own the sheep are not, seeth the wolf coming, and leaveth the sheep, and fleeth: and the wolf catcheth them, and scattereth the sheep.
13 The hireling fleeth, because he is an hireling, and careth not for the sheep.

Among those who owned sheep, some kept their own flocks and some hired shepherds to care for them. Every owner must naturally have felt more interested in the preservation of his flock than the hireling could possibly feel. The hireling counts the sheep his own as long as they are profitable to him. The good shepherd looks upon them as his, as long as he can be profitable to them.

14 I am the good shepherd, and know my sheep, and am known of mine.

[I...know my sheep] I know them that are mine: their hearts, their wishes, their intentions, their circumstances; and I approve of them; for in this sense the word to know is often taken in the scriptures. Dedicated shepherds became so acquainted with their individual sheep that, even when mixed in with others, they could easily distinguish them.

[And am known of mine] They have me as their father, protector and savior. They acknowledge me and my truth before the world, and they approve of me by their attachment to me and their zeal for my glory.

15 As the Father knoweth me, even so know I the Father: and I lay down my life for the sheep.

God, the Father, and God the Son are of one mind. The wages of sin is death, so Father and Son agree to make that payment on behalf of the sheep.
"For all have sinned, and come short of the glory of God." Romans 3:23.
"For the wages of sin is death, but the gift of God is eternal life through Jesus Christ, our Lord." Romans 6:23.
"For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life." John 3:16.
"And every priest standeth daily ministering and offering often the same sacrifices, which can never take away sins; But this man, after he had offered one sacrifice for sins forever, sat down on the right hand of God, from henceforth, expecting till his enemies be made his footstool." Hebrews 10:11-13.

16 And other sheep I have, which are not of this fold: them also I must bring, and they shall hear my voice; and there shall be one fold, and one shepherd.

[Other sheep I have] The Gentiles and Samaritans.

17 Therefore doth my Father love me, because I lay down my life, that I might take it again.

[Therefore doth my Father love me] As I shall shortly be crucified by you, do not imagine that I have been abandoned by my heavenly Father. The Father loves me particularly on this account, because I am going to lay down my life for the life of this world. Again, do not suppose that I shall be put to death by your rulers because I do not have the strength to resist them. I lay down my life voluntarily and cheerfully; no one can take it away from me; and I shall give you the fullest proof of my supreme power by raising, in three days, that very crucified body from the tomb.

18 No man taketh it from me, but I lay it down of myself. I have power to lay it down, and I have power to take it again. This commandment have I received of my Father.

[There was a division] They were divided in their opinions, one part received the light, others resisted it. Jesus had given them an infallible proof to anticipate. He would rise again.