One reason why many commentators have been hesitant to speak about an overarching theology in Numbers is the presence in Numbers of genres of literature that are not only radically distinct, but often ones that seem at first sight to be arranged almost haphazardly.
I would suggest that all of these disparate materials revolve around the theme of holiness; specifically, the holiness mandated of a community as it prepares to break camp (1:1-10:10), and the holiness expected of that community as it heads toward the full geographical and spiritual fulfillment of God's will for its future (10:11-36:13). Childs (p. 199) comments, “In spite of its diversity of subject matter and complex literary development, the book of Numbers maintains a unified sacerdotal interpretation of God's will for his people which is set forth in a sharp contrast between the holy and the profane.”
The “holy” is, of course, the blessing presence of God (6:22-27) in the midst of his people as they change from campers to marchers, and his expectation that those whom he blesses will live distinct, separated lives. There are, however, innumerable instances cited in Numbers which would, if allowed to happen, call into question that uniqueness.
On one hand, 1:10-10:10 deals with potential threats to that holiness: uncleanness, deliberate sin, infidelity, violation of a vow, and presumptuous observance of sacred festivals.
Numbers 11:1-25:18, on the other hand, describe actual threats to that holiness. For the most part, the greatest threat lies within the sphere of relationships within the community. There are a number of narratives here (chs. 11-14, 16) that detail a murmuring, back-stabbing, strife-fomenting group of malcontents. Is this the way God's redeemed and set-apart people are to live? How can the Lord bless, keep, and make his face shine on such troublemakers? He cannot, for the sacred character of the community is impugned when the fabric of that community is torn by dissension and nitpicking. It compels God to move from blessing to judgment. He turns the spell of Balaam into a blessing, but he does not do the same with Israel's penchant for divisiveness or her lapse into sexual debauchery.
In some instances God may have to give up on an entire generation (and even its leaders, viz., Moses and Aaron) and regroup with a second generation. But for this new generation the standard of holiness is not diminished. They too live under the possibility of blessing or judgment. They too must choose between cleanness and defilement, between holy living and autonomy. God is committed transgenerationally both to the perpetuation of Israel's existence and to the nonnegotiable standard of holiness by which any generation of his followers must live.