19 Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you.

[The doors were shut...for fear of the Jews] We do not find that the Jews intended to molest the disciples: that word of authority which Christ spoke, John 18:8, "Let these go their way," had prevented the Jews from offering them any injury; but, since the Jews had proceeded so far as to put Christ to death, and since the faith of the disciples was not very strong, they were afraid that they would be the next victims if found.

The evangelist has omitted the appearing of our Lord to the other women who came from the tomb, Mark 16:1, and to the two disciples who were going to Emmaus, Luke 24:13, which all happened on the same day.

20 And when he had so said, he shewed unto them his hands and his side. Then were the disciples glad, when they saw the LORD.

[He showed them his hands and his side] His body bore the marks of the nails and the spear, and these marks were preserved that the disciples might be the more fully convinced of the reality of his resurrection.

21 Then said Jesus to them again, Peace be unto you: as my Father hath sent me, even so send I you.

[Even so send I you] As I was sent to proclaim the truth of the Most High, and to convert sinners to God, I send you for the very same purpose, clothed with the same authority and influenced by the very same Spirit.

22 And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost:

[He breathed on them] Intimating, by this, that they were to be made new men in order to be qualified for the work to which he had called them: for in this breathing he evidently alluded to the first creation of man, when God breathed into him the breath of life, and he became a living soul. Genesis 2:7.

23 Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained.

[Whose soever sins ye remit] It is certain that God alone can forgive sins and it would be not only blasphemous, but grossly absurd, that any creature could remit the guilt of a transgression which had been commited against the Creator. The apostles received from the Lord the doctrine of reconciliation and the doctrine of condemnation. Those who believed on the Son of God in consequence of the apostle's preaching, had their sins remitted; and those who would not believe were declared to be under condemnation. John 3:14-18.

24 But Thomas, one of the twelve, called Didymus, was not with them when Jesus came.
25 The other disciples therefore said unto him, We have seen the LORD. But he said unto them, Except I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe.

[I will not believe] His unbelief was (1) unreasonable. Ten of his brethren witnessed that they had seen Christ, but he rejected their testimony. (2) Obstinate. He was determined not to believe on any evidence that God might choose to give him. He would believe according to his own rules, or not at all. (3) Presumptious and insolent. A view of the person of Christ would not suffice. He would not believe unless he could put his finger into the holes made by the nails or thrust his hand into the wound made by the spear.

26 And after eight days again his disciples were within, and Thomas with them: then came Jesus, the doors being shut, and stood in the midst, and said, Peace be unto you.
27 Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing.
28 And Thomas answered and said unto him, My LORD and my God.

[My Lord and my God] Jesus condescended to give Thomas the opportunity which he requested, but when face to face with the risen Son of God his doubts evaporated.

29 Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed.

[Blessed are they...] You have seen and therefore have believed, and now you are blessed, you are happy -- now fully convinced of my resurrection. Blessed also are those who believe the evidence which I provide them through the testimony of the apostles and their Spiritual descendents.

30 And many other signs truly did Jesus in the presence of his disciples, which are not written in this book:
31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.

The express purpose the Holy Spirit had in mind when leading John to create this document.