Question: Who led Jesus' baptism in the Gospel of Mark?

Context paragraphs:
Infant baptism: Infant baptism is the practice of baptising infants or young children.  In theological discussions, the practice is sometimes referred to as paedobaptism, or pedobaptism, from the Greek "pais" meaning "child".  The practice is sometimes contrasted with what is called "believer's baptism", or credobaptism, from the Latin word "credo" meaning "I believe", which is the religious practice of baptising only individuals who personally confess faith in Jesus, therefore excluding underage children.  Opposition to infant baptism is termed catabaptism.  Infant baptism is also called "christening" by some faith traditions.

Baptism: Baptism (from the Greek noun βάπτισμα "baptisma"; see below) is a Christian sacrament of admission and adoption, almost invariably with the use of water, into the Christian Church generally.  The canonical Gospels report that Jesus was baptized—a historical event to which a high degree of certainty can be assigned.  Baptism has been called a holy sacrament and an ordinance of Jesus Christ.  In some denominations, baptism is also called christening, but for others the word "christening" is reserved for the baptism of infants.  Baptism has also given its name to the Baptist churches and denominations.

History of baptism: The Christian rite of baptism has similarities to Tevilah, a Jewish purification ritual of immersing in water which is required for, among other things, conversion, but differs in that Tevilah is repeatable, while baptism is to be performed only once.  John the Baptist, who is considered a forerunner to Christianity, used baptism as the central sacrament of his messianic movement.  Christians consider Jesus to have instituted the sacrament of baptism.  The earliest Christian baptisms were probably normally by immersion, though other modes may have also been used.  By the third and fourth centuries, baptism involved catechetical instruction as well as chrismation, exorcisms, laying on of hands, and recitation of a creed.  Affusion became the normal mode of baptism between the twelfth and fourteenth centuries, though immersion was still practiced into the sixteenth.  In the sixteenth century, Martin Luther retained baptism as a sacrament, but Swiss reformer Huldrych Zwingli considered baptism and the Lord's supper to be symbolic.  Anabaptists denied the validity of infant baptism, which was the normal practice when their movement started and practiced believer's baptism instead.  Several other groups, notably the Baptists and Dunkards, have always practiced baptism by immersion as following the Biblical example.

Baptism of Jesus: The baptism of Jesus is described in the gospels of Matthew, Mark and Luke.  John's gospel does not directly describe Jesus' baptism.

Baptism of the Lord: The Baptism of the Christ (or the Baptism of Christ) is the feast day commemorating the baptism of Jesus in the Jordan River by John the Baptist.  Originally the baptism of Christ was celebrated on Epiphany, which commemorates the coming of the Magi, the baptism of Christ, and the wedding at Cana.  Over time in the West, however, the celebration of the baptism of the Lord came to be commemorated as a distinct feast from Epiphany.  It is celebrated in the Roman Catholic Church as well as the Anglican and Lutheran Churches on the first Sunday following The Epiphany of Our Lord (6 January).

John the Baptist: John the Baptist (Ancient Greek: Ἰωάννης ὁ βαπτιστής , "Ioánnes (h)o baptistés" or Ἰωάννης ὁ βαπτίζων, "Ioánnes (h)o baptízon", known as the Prophet Yahya in the Quran), also known as John the Baptizer, was a Jewish itinerant preacher in the early first century AD.  John is revered as a major religious figure in Christianity, Islam, the Bahá'í Faith, and Mandaeism.  He is called a prophet by all of these traditions, and is honoured as a saint in many Christian traditions.

Gospel of the Hebrews: The Gospel of the Hebrews (Greek: "τὸ καθ' Ἑβραίους εὐαγγέλιον" ), or Gospel according to the Hebrews, was a syncretic Jewish–Christian gospel, the text of which is lost; only fragments of it survive as brief quotations by the early Church Fathers and in apocryphal writings.  The fragments contain traditions of Jesus' pre-existence, incarnation, baptism, and probable temptation, along with some of his sayings.  Distinctive features include a Christology characterized by the belief that the Holy Spirit is Jesus' Divine Mother and a first resurrection appearance to James, the brother of Jesus, showing a high regard for James as the leader of the Jewish Christian church in Jerusalem.  It was probably composed in Greek in the first decades of the 2nd century, and is believed to have been used by Greek-speaking Jewish Christians in Egypt during that century.

Epiphany (holiday): Epiphany ( ), also Theophany or Three Kings' Day, is a Christian feast day that celebrates the revelation of God incarnate as Jesus Christ.  In Western Christianity, the feast commemorates principally (but not solely) the visit of the Magi to the Christ child, and thus Jesus' physical manifestation to the Gentiles. Moreover, the feast of the Epiphany, in some Western Christian denominations, also initiates the liturgical season of Epiphanytide.  Eastern Christians, on the other hand, commemorate the baptism of Jesus on both sides of the Jordan River, seen as his manifestation to the world as the Son of God.  Qasr el Yahud in the West Bank, and Al-Maghtas in Jordan on the east bank, Al-Maghtas is considered to be the original site of the baptism of Jesus and the ministry of John the Baptist.

Conditional baptism: Mainline Christian theology (including Roman Catholic, Eastern Orthodox, Oriental Orthodox, Church of the East, Anglican, Lutheran and most other Protestants) has traditionally held that only one baptism is valid to confer the benefits of this sacrament.  In particular, the Council of Trent defined a dogma that it is forbidden to baptize a person who is already baptized, because the first baptism would make an indelible mark on the soul.  Likewise, "Methodist theologians argued that since God never abrogated a covenant made and sealed with proper intentionality, rebaptism was never an option, unless the original baptism had been defective by not having been made in the name of the Trinity."  Therefore, in cases where the validity of a baptism is in doubt, a "conditional" baptism may be performed.

Gospel of Mark: The Gospel According to Mark (Greek: τὸ κατὰ Μᾶρκον εὐαγγέλιον , "to kata Markon euangelion"), the second book of the New Testament, is one of the four canonical gospels and one of the three synoptic gospels.  It tells of the ministry of Jesus from his baptism by John the Baptist to his death and burial and the discovery of the empty tomb – there is no genealogy of Jesus or birth narrative, nor, in the original ending at chapter 16, any post-resurrection appearances.  It portrays Jesus as a heroic man of action, an exorcist, a healer, and a miracle worker.  Jesus is also the Son of God, but he keeps his identity secret, concealing it in parables so that even the disciples fail to understand.  All this is in keeping with prophecy, which foretold the fate of the messiah as suffering servant.  The gospel ends, in its original version, with the discovery of the empty tomb, a promise to meet again in Galilee, and an unheeded instruction to spread the good news of the resurrection.

Supporting paragraphs: Gospel of Mark John the Baptist
Answer: John the Baptist