By stepping into the place of sinners (Pope Benedict XVI)

  Looking at the events [of Christ’s baptism] in the light of the Cross and Resurrection, the Christian people realized what happened:  Jesus loaded the burden of all mankind’s guilt upon his shoulders; he bore it down into the depths of the Jordan.  He inaugurated his public activity by stepping into the place of sinners.  His inaugural gesture is an anticipation of the Cross.  He is, as it were, the true Jonah who said to the crew of the ship, Take me down and throw me into the sea (Jonah 1:12) … The baptism is an acceptance of death for the sins of humanity, and the voice that cries out, This is my beloved Son over the baptismal waters is an anticipatory reference to the Resurrection.  This also explains why, in his own discourses, Jesus uses the word baptism to refer to his death.

  The icon of Jesus’ baptism depicts the water as a liquid tomb having the form of a dark cavern, which is in turn the iconographic sign of Hades, the underworld, or hell.  Jesus’ descent into this watery tomb, into this inferno that envelops him from every side, is thus an anticipation of his act of descending into the underworld… John Chrysostom writes, Going down into the water and emerging again are the image of the descent into hell and the Resurrection.

--Pope Benedict XVI, Jesus of Nazareth

Image source (1 & 2):  Baptism of Christ, Theophany Icon, https://iconreader.wordpress.com/2011/01/06/baptism-of-christ-the-theophany-icon/