Owing largely to Reformation-era polemics, the application of legal descriptions in the context of historical Christian orthodoxy appears to have been badly misunderstood. The charge is that the Eastern Fathers were pretty much oblivious to legal metaphors, the Western Fathers were cognizant but confused owing to translation errors (specifically the use of iustificare, to make righteous), and only the discussion around the Reformation uncovered the true Biblical teaching about what it meant to be declared righteous. I think that's oversimplified, so I will try to put a more helpful spin on what purpose legal descriptions serve.
The Eastern view deals with the question of God's rule rather easily: the reign (basileia) of God is nothing other than His divine power, glory and grace, viz., the uncreated divine energies. The reason that the Eastern Fathers spend little time on concepts like justification is that there is really no need for it. The personal interaction between God and man is based on participation in the divine energies, and this in turn serves as the law of Christ. Obviously, the account of participation in the divine energies was extensively developed, so there was no purpose in a separate theology of law. The use some or another image, like legal imagery, has no intrinsic significance; it is simply another sign used to point to the concept.
While this is entirely sensible and even technically correct, it's a little unsatisfying. Given the rich history behind this judicial imagery in the Old Testament and its extensive use in the New Testament (especially justification, adoption, and redemption), a guy's gotta wonder at least why these particular images are used. And in particular, the reigns of God and Satan are described in opposition to one another, and Satan clearly doesn't have uncreated divine energies or divine power, so there's a broader concept at work here.
I would propose that the legal descriptions are closest to the idea of citizenship. But it's not citizenship in the sense of being either on God's team or Satan's team, because there are citizens of God's kingdom who end up working for Satan. Rather, it's the idea that the authority under which one lives determines the applicable laws. And this makes sense in the story of the Bible: Adam starts out in Eden in communion with God, then he is deprived of God's grace, placing him in Satan's kingdom. The Law is given to teach the coming of Christ, who establishes the kingdom of God. On the last day, the kingdom of God conquers the kingdom of Satan, and everybody ends up being citizens of God's kingdom.
Those born after Adam are born in the kingdom of Satan, and the rules there make a gulag in Siberia look like a day on the beach. You get to die. You can't please God by anything you do, so not only to you get to die, you get to spend an eternity in Sheol with no prospect of ever reaching God. And here's the best part: if you do anything wrong in your whole entire life, you also win eternal torment. There are only two ways out of the kingdom of Satan: asylum or the invasion on the last day. If you get out on the last day, there is good news and bad news. The good news is that you get your body back (yay!), because one of the rules in God's kingdom is that everybody gets resurrected. The bad news is that you are still tormented for all eternity, except now you get bodily torment as well (boo!).
If I have properly delivered the Good News, you should now be asking "that asylum deal sounds pretty good, so how can I get on that train?" This is the part that I argue the legal descriptions are intended to convey. You get a legal declaration that the rule of Satan no longer applies to you and that what you have done in his kingdom if forgiven; we call this initial justification. You are judged ransomed and redeemed, and Satan no longer has any reign over you except for what you give him. You get a metaphysical tattoo (a character or stamp) that can never be taken away marking you as a citizen of God's kingdom. You are adopted as a son of God, because all of the willing citizens of the Kingdom of God are brothers. You have an inheritance and a place prepared for you in Heaven, although it may stand as a judgment against you if you are not worthy to claim it.
Yes, the Kingdom of God has rules too. Asylum is a "get out of Satan's kingdom free" card (i.e., without works), but not a "get out of Hell free" card. Paul describes himself as "not being without law toward God but under the law of Christ" (1 Cor. 9:21, cf. Gal. 6:2, Matt. 22:35-40). And woe to the one who breaks the Law of Christ: "How much worse punishment do you think will be deserved by the man who has spurned the Son of God, and profaned the blood of the covenant by which he was sanctified, and outraged the Spirit of grace?" (Hebr. 10:29). So the legal declaration that makes you a citizen of the Kingdom of God gives you the rights and responsibilities of a willing citizen, but you still have to be a good and productive citizen (i.e., maintain your faith working in love) to retain those benefits. Otherwise, the law of the Kingdom stands as a judgment against you.
