Tetrarch, from the Greek words tetra, meaning four, and arche, meaning rule, was used by the Greeks for a commander of a section of a phalanx (a dense infantry battle formation of swordsmen and spearmen used to penetrate and overrun a less-concentrated enemy force). The Romans later used the title for a Roman governor over a fourth of a province. The term became practically used for a ruler of a province generally, with the tetrarch also sometimes referred to as a king. The most-mentioned tetrarch of Bible History is Herod Antipas (see The Herods) who was directly involved in the deaths of both John The Baptist and Jesus Christ (it was the father of Herod Antipas, Herod The Great, who tried to have Christ killed as an infant).
Along with the Caesar and lesser governors, the Scriptures record a number of tetrarchs in the New Testament era:
"In the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate being governor of Judea, and Herod being tetrarch of Galilee, and his brother Philip tetrarch of the region of Ituraea and Trachonitis, and Lysanias tetrarch of Abilene, in the high-priesthood of Annas and Caiaphas, the word of God came to John the son of Zechariah in the wilderness; and he went into all the region about the Jordan, preaching a baptism of repentance [see also The Origin of Baptism] for the forgiveness of sins." (Luke 3:1-3 RSV)
It was the tetrarch Herod Antipas who ordered the beheading of John the Baptist at Machaerus:
"John answered them all, "I baptize you with water; but he who is mightier than I is coming, the thong of whose sandals I am not worthy to untie; he will baptize you with the Holy Spirit and with fire [see Baptism Of Fire]. His winnowing fork is in his hand, to clear his threshing floor, and to gather the wheat into his granary, but the chaff he will burn with unquenchable fire." So, with many other exhortations, he preached good news to the people. But Herod the tetrarch, who had been reproved by him for Herodias, his brother's wife, and for all the evil things that Herod had done, added this to them all, that he shut up John in prison." (Luke 3:16-20 RSV)
Herod Antipas also knew of Jesus of Nazareth, and, like John, that He too was a man of God (a known reality that nevertheless did not prevent him from having both of them killed):
"At that time Herod the tetrarch heard about the fame of Jesus; and he said to his servants, "This is John the Baptist, he has been raised from the dead; that is why these powers are at work in him." (Matthew 14:1-2 RSV)
The Messiah faced the tetrarch Herod Antipas during the night before the Crucifixion:
"When Pilate heard this, he asked whether the man was a Galilean. And when he learned that He belonged to Herod's jurisdiction, he sent Him over to Herod, who was himself in Jerusalem at that time. When Herod saw Jesus, he was very glad, for he had long desired to see Him, because he had heard about Him, and he was hoping to see some sign done by Him. So he questioned Him at some length; but He made no answer. The chief priests and the scribes stood by, vehemently accusing Him. And Herod with his soldiers treated Him with contempt and mocked Him; then, arraying Him in gorgeous apparel, he sent Him back to Pilate. And Herod and Pilate became friends with each other that very day, for before this they had been at enmity with each other." (Luke 23:6-12 RSV)
Fact Finder: What was the name of the man who was formerly a member of the court of Herod the tetrarch (by that time likely meaning Herod Agrippa, see The Herods) who came to be among the Christian prophets and teachers?