And he called to him the twelve, and began to send them out two by two, and gave them authority over the unclean spirits. He charged them to take nothing for their journey except a staff; no bread, no bag, no money in their belts; but to wear sandals and not put on two tunics. And he said to them, "Where you enter a house, stay there until you leave the place. And if any place will not receive you and they refuse to hear you, when you leave, shake off the dust that is on your feet for a testimony against them." So they went out and preached that men should repent. And they cast out many demons, and anointed with oil many that were sick and healed them.
King Herod heard of it; for Jesus' name had become known. Some said, "John the baptizer has been raised from the dead; that is why these powers are at work in him." But others said, "It is Elijah." And others said, "It is a prophet, like one of the prophets of old." But when Herod heard of it he said, "John, whom I beheaded, has been raised." For Herod had sent and seized John, and bound him in prison for the sake of Herodias, his brother Philip's wife; because he had married her. For John said to Herod, "It is not lawful for you to have your brother's wife." And Herodias had a grudge against him, and wanted to kill him. But she could not, for Herod feared John, knowing that he was a righteous and holy man, and kept him safe. When he heard him, he was much perplexed; and yet he heard him gladly.
But an opportunity came when Herod on his birthday gave a banquet for his courtiers and officers and the leading men of Galilee. For when Herodias' daughter came in and danced, she pleased Herod and his guests; and the king said to the girl, "Ask me for whatever you wish, and I will grant it." And he vowed to her, "Whatever you ask me, I will give you, even half of my kingdom." And she went out, and said to her mother, "What shall I ask?" And she said, "The head of John the baptizer." And she came in immediately with haste to the king, and asked, saying, "I want you to give me at once the head of John the Baptist on a platter." And the king was exceedingly sorry; but because of his oaths and his guests he did not want to break his word to her. And immediately the king sent a soldier of the guard and gave orders to bring his head. He went and beheaded him in the prison, and brought his head on a platter, and gave it to the girl; and the girl gave it to her mother.
When his disciples heard of it, they came and took his body, and laid it in a tomb. 
The duty of Christians to take part in the life of the Church impels them to act as witnesses of the Gospel and of the obligations that flow from it. This witness is a transmission of the faith in words and deeds. Witness is an act of justice that establishes the truth or makes it knownFrom this definition of martyrdom in the Catechism we can identify a number of different elements the most fundamental of which are the elements of witness to the person of Christ and the fact that this witness leads to death of the person giving witness. After these two elements, however, there are a number of different ways in which martyrdom is manifested in the life of the Church.
Martyrdom is the supreme witness given to the truth of the faith: it means bearing witness even unto death. The martyr bears witness to Christ who died and rose, to whom he is united by charity. He bears witness to the truth of the faith and of Christian doctrine. He endures death through an act of fortitude.
In the case of John the Baptist, the direct witness that he gave that led to his death was a witness to fidelity in marriage. It was his condemnation of Herodias relationship with Herod that provoked the anger of Herodias, and led her to ask for his death. This is a witness to a particular teaching about Christian living, a particular point of Christian doctrine.
We can suggest a parallel between this witness to teaching about fidelity in marriage and the wedding feast at Cana, which in St John’s Gospel, foreshadows the events of the Last Supper and Calvary as the “wedding feast of the Lamb”. The witness to fidelity in marriage is therefore much more profoundly a witness to the person and mission of Jesus than at first appears.
This can be further developed if we see in the structure of St Mark’s account of the death of John the Baptist a foretelling of Jesus’ own passion and death. This can be done by comparing the account of his death to the passion account in St Mark’s Gospel. John the Baptist’s martyrdom then follows integrally from his mission as the forerunner of Jesus:
Herod is wrong about John being raised up from the dead; Jesus will rise from the dead (Mk 16:6-8)This understanding recognises, too, that all martyrdom is aligned towards and gains its meaning from the death of Jesus himself on the Cross.
Herodias had to be devious about achieving John’s death, because of the respect that Herod had for him; the Chief priests will also have to be devious in bringing about Jesus death because of what the people might do (Mk 11:18, 14:1-2)
Herod has John beheaded even though he knows that he has done no wrong, under pressure from Herodias and his guests; Pilate will have Jesus put to death, though he knows he has done no wrong, and acting under pressure from the chief priests and the people (Mk 15:14)
John’s disciples lay his body in a tomb; a disciple of Jesus will lay his body in a tomb (Mk15:46)
 Mk 6:7-27; cf Mt 14:1-12 (in this shorter account it is Jesus’ own preaching that comes directly to the attention of Herod, rather than that of the twelve; in both accounts, however, it is essentially Jesus reputation that is involved.); St Luke records only the imprisonment of John the Baptist Lk 3:19-20.
 Catechism of the Catholic Church nn.2472-2473.
 cf The Collegeville Bible Commentary p.916.
 cf The Collegeville Bible Commentary p.916