Jesus is Rider on White Horse. Pope Pius XII

Taken from:
http://www.catholicbinder.com/WebPages/TheWarning/TheSevenSeals.aspx

THE FIRST SEAL

Then I watched while the Lamb broke open the first of the seven seals, and I heard one of the four living creatures cry out in a voice like thunder, "Come forward." I looked, and there was a white horse, and its rider had a bow. He was given a crown, and he rode forth victorious to further his victories. (6:1-2)

This Rider, according to Sacred Tradition, is the Lord Himself:

…of whom also John says in the Apocalypse: "He went forth conquering, that He should conquer." —St. Irenaeus, Against Heresies, Book IV: 21:3
[The Rider] is Jesus Christ. The inspired evangelist [St. John] not only saw the devastation brought about by sin, war, hunger and death; he also saw, in the first place, the victory of Christ.—POPE PIUS XII, Address, November 15, 1946; footnote of The Navarre Bible, "Revelation", p.70

Jesus is seen in this vision preceding the other "riders" of the Apocalypse that will follow in the other seals. What are the victories He achieves?

The first seal being opened, he says that he saw a white horse, and a crowned horseman having a bow. For this was at first
done by Himself. For after the Lord ascended into heaven and opened all things, He sent the Holy Spirit, whose words the preachers sent forth as arrows reaching to the human heart, that they might overcome unbelief. —St. Victorinus, Commentary on the Apocalypse,Ch. 6:1-2

That is, mercy is preceding justice. This is precisely what Jesus announced through His "secretary of mercy," St. Faustina:

…before I come as the just Judge, I am coming first as the King of Mercy… before I come as a just Judge, I first open wide the door of My mercy. He who refuses to pass through the door of My mercy must pass through the door of My justice… —Divine Mercy in My Soul, Jesus to St. Faustina, Diary, n. 83, 1146

These victories are to be achieved throughout the spiral of history untilthe cup of justice is full.² The final "arrows" shot from the bow of this Rider are the last words of invitation to repent and believe the good news—the beautiful and consoling message of Divine Mercy³—before the other riders of the apocalypse begin their final gallop throughout the world.

Today, a living flame of divine love entered my soul… It seemed to me that, if it had lasted an instant longer, I would have been drowned in the ocean of love. I cannot describe these arrows of love that pierce my soul.—Divine Mercy in My Soul, Jesus to St. Faustina, Diary, n. 1776

While these messages are being heeded today by some souls throughout the world, it has not been enough to stem the moral tsunami that has produced a culture of death…

Mankind has succeeded in unleashing a cycle of death and terror, but failed in bringing it to an end… —POPE BENEDICT XVI, Homily Esplanade of the Shrine of Our Lady
of Fátima, May 13th, 2010