What is "the Law of Christ"? How do we define it? We find such phrases in the NT, but we don't find a "legal code" such as existed under the Old Covenant.
What do we find? We find moral instruction from Jesus himself in the gospels, and his teaching (doctrine and practice) as expressed by the apostolic witness to him. We have Jesus himself saying such things as "I came not to destroy the Law, but to fulfil" (Mt. 5:17). We have Paul in 1 Tim. 1:8-11 basically appealing to the 10 commandments as he proclaims the right use of law, to go against which is "contrary to sound doctrine."
So we have two things: 1) the end of the Law, and 2) the continuation of law. We have one and the same Law-giver. We have the continuing relevance and condemnation of humanity according to the original Covenant of Works (Rom. 1:28-32; 2:11-16).
The Moral Law is summarily comprehended in the 10 Commandments. As the Confession puts it, the original law was put into or included as integral or foundational to the Mosaic Law (19.2). So simply putting the frame of the Mosaic Law aside does not nullify or abrogate the Moral Law. Nor do the 10 Commandments cease to be a faithful summary of that more essential Moral Law simply because the Law as a whole has been set aside.
As WCF 19.5 puts it, "The moral law doth for ever bind all, as well justified persons as others, to obedience thereof." We are not bound by the 10 Commandments per se, but we are bound by the 10 Commandments as a summary of the Moral Law of God.
It is for this reason that Paul and the other NT writers turn back to the OT law without any sense of awkwardness as an expression of righteousness and judgment against sinners. There is no need for a new "code". The old one will suffice for all current purposes. For everyone who is not under the New Covenant is already (still) condemned by the Old.
And the New Covenant adminstration has no interest in abolishing the relevance either of history or of instruction in law as it pertains to its present members. As well, the law "is of great use to them, ... in that, as a rule of life informing them of the will of God, and their duty, it directs, and binds them to walk accordingly; discovering also the sinful pollutions of their nature, hearts, and lives; so as, examining themsleves thereby, they may come to further conviction of, humiliation for, and hatred against sin..." (19.6). There is additional use published in that paragraph.
The bottom line is, we only know what is the "Law of Christ" or the "Law of Love" by recourse to the whole revelatory Bible. We have no "code" precisely because this is the New Covenant Age, Christ has come, the Moral Law is being refashioned upon our hearts, and a "code" is out of place in this era. Out of place in the sense that "there is therefore now no condemnation to those who are in Christ Jesus." Our union with Christ renders a "code" (eventually!) superfluous.
But in the flesh we still need an objective reference. As church governors, we still need an objective standard that holds men accountable. As individuals we still need to check ourselves. What are we going to use, if not some expression of the Moral Law?
Anyway, I hope this puts some meat on the bones...
{Edit}
Let me add, with regard to "indicative/imperative": Law is not merely an historic relic, and therefore descriptive and not prescriptive. There is still Law (the words "Law of Love" prove that!). There is still the principle of law at work forever. Where there is a God and moral, creaturely agents, there is Law as an expression of divine will. Law by definition is prescriptive where it is actively in force.
Thus imperatively the law is very much alive. How many Christians do you know who's lives are "models" of the Law, who are sinless (1 Jn. 1:8 ! ). Indicitavely, does the Law describe me? I wish! How is my "lawlessness" (1 Jn. 3:4) discovered and defined? We continually have recourse to the Bible to reveal the moral will of God.
It is "the gospel" that begins with the "triumphant indicative." This is why we need constantly go back to the gospel, after our Law-failures. For without the gospel, the heart that condemned us (1 Jn. 3:20) could never be made confident again (v. 21). We could never (v. 22) ask anything because we "keep his commandments," which is only possible (v. 23) by faith.
Law and Gospel are inseperable.
[Edited on 4-13-2006 by Contra_Mundum]