From The Gospel of Mark, by Mary Healy, commenting on Mark 1:14-20
The kingdom of God is a favorite theme in the Synoptics and the most characteristic term Jesus uses to signify what he is about. Later he will unfold its meaning in a series of parables (4:1–32). Although this phrase never appears in the Old Testament, it sums up Israel’s yearning for the full manifestation of God’s authority in Israel and in the whole world: “The Lord of hosts will reign” (Isa 24:23; see 52:7; Zech 14:9).
Jesus’ announcement that the kingdom is at hand suggests both a present and a future quality, like a sunrise below the horizon. The kingdom is already present, embodied in Jesus’ own person. Indeed, throughout his ministry it will become evident that the “foreign occupation” of sin, Satan, disease, and death is being overthrown. Yet the kingdom is incipient and partly veiled; like seeds sown in the ground, it will keep growing until it reaches its consummation (4:26–29).
The arrival of the kingdom calls for a twofold human response: to repent, and believe in the gospel. Jesus is taking up a theme of the prophets: God’s continual call for his people to repent or “turn back” to him with all their hearts (Neh 1:9; Isa 44:22; Hosea 14:2). The Baptist had already begun to sound this call (v. 4). But Jesus adds a new accent with the invitation to believe, that is, trustingly accept and yield to what God is doing in him. The kingdom is near enough that anyone who so chooses can reach out and lay hold of it through faith.