Prosecution of the Body of Christ

And Saul, yet breathing out threatenings and slaughter against the disciples of the Lord, went unto the high priest, And desired of him letters to Damascus to the synagogues, that if he found any of this way, whether they were men or women, he might bring them bound unto Jerusalem. And as he journeyed, he came near Damascus: and suddenly there shined round about him a light from heaven: And he fell to the earth, and heard a voice saying unto him, Saul, Saul, why persecutest thou me?
- Acts 9:1-4 (KJV)

Note that Jesus says that persecuting Christians is the same as persecuting Jesus Himself.

That is because together we are the body of Christ.

And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.
- Colossians 1:18 (KJV)

Saul was hurting the body of Christ, and that's why Jesus stopped him.

And if one member suffers, all the members suffer with it; or if one member is honored, all the members rejoice with it.
- 1 Corinthians 12:26 (KJV)