The first fifteen verses of Mark comprise one of my favorite passages of scripture, because there are so many valuable treasures of theological truth to be found here. I want to focus merely on Jesus' preaching as recorded in v. 15: The time has come, the kingdom of God has come near. Repent and believe the good news!
I posted recently about Jon Levenson's insights regarding the dialectic of divine sovereignty in the Old Testament. Briefly stated, Levenson posits that a part of OT Jewish theology is a simultaneous affirmation, denial, and hope of the sovereignty or rule of God on earth: God had accomplished great feats, displayed an impressive mastery over the world in times past (e.g., in the creation, in the exodus); but as things stand, the world is in disarray and everything is falling apart, chaos rules and God's law is not obeyed; consequently we hope for and invoke the arrival of God's kingdom, his sovereignty, his rule, when the disorder of the world will be made orderly once more.
In light of this view, what else can Jesus' message of the kingdom of God be except a critically eschatological one: now, after all the time which has past, God is finally beginning to take rule of the world; things are finally going to go his way, his reign will finally be materialized.
At his baptism, in analogy to Old Testament coronation of the kings (cf. Ps 2.7), Jesus is told by God: "You are my Son!" Just as the prophet would have anointed an Israelite king with oil, so Jesus is anointed in baptism and installed as king by John the Baptizer. Therefore, it is important to understand that this kingdom is localized in Jesus' own person: it is he who obeys God's will, who fulfills God's expectations for Israel (Mt 5.17-8) and for humanity as a whole (Col 1.15; cf. Gen 1.26-7 LXX). In Jesus Christ, God's desires and goals for the creation are realized and fulfilled: in Jesus Christ, the eschaton is now!
Yet this eschaton is oriented ad extra, outwards, to bring others in. God evidently is not satisfied with, and his whole will is not merely the realization of his sovereignty in Jesus; for God in Jesus preaches that others repent and believe this fantastically good news, this evangelion that God is finally straightening what is crooked in the world through Jesus Christ. Repentance and belief are the means by which one accesses and participates in this eschaton-brought-near; and through these means, oneself is conformed to God's goals and desires for the world, so that we too begin to embody and materialize God's reign on earth.
In brief, then, the announcement of the kingdom of God, localized in Jesus' person but available to others through faith and repentance, is no less than the affirmation of the realization of OT eschatological hopes: finally, God is sovereign; finally, his reign is materializing in this disordered and chaotic world, and all are given the invitation to come in and enjoy this.