One way to explain imputation is to say it is the based on a “legal union” between the believer. This seems problematic to me because it is merely a restatement of the basic fact, not an explanation. What is legal union? Why it is a relationship that justifies imputation.

For legal union to work, it needs to be presented as having a basis.

Would it have been perfectly OK for God to simply declare Adam the legal representative of Gabriel? Would it have been just of God to impute Adam’s sin to him? Think of it. Gabriel arrives back in heaven after completing a reconnaissance mission in the Deneb star system and finds a legion of seraphim with flaming swords barring his path. “What is going on?” he asks.

“Get back, sinner!” cries the commanding seraph. “No sinner is permitted beyond this threshhold.”

“But I’ve never sinned!” Gabriel says. “You are unjustly accusing me.”

“No, no,” replies the commander. “We’re not simply accusing you? Adam sinned while you were gone and God effected a legal union between him and you. So you are guilty and there is nothing unjust about it because of imputation.

“Wow,” exclaims Gabriel. “That explains everything. I was wrong to accuse you of injustice. Please forgive me.”

“Can’t.  You’re a sinner. Sinner’s can’t be forgiven.”

A sticking point in explaining the need for and role of imputation in the salvation of sinners is that it is supposed to have apologetic value.  It is supposed to explain how a just judge can declare the guilty to be righteous.  God cannot, we say, simply forgive sinners without any other consideration.  Something must happen to make the transaction truly just and right.

But, if imputation itself is portrayed in a way that seems arbitrary and imaginary then it ceases to have any apologetic value.  How is God’s simply forgiving sinners less problematic than His imagining that one person’s righteousness belongs to another who is unrighteous in himself?  And when considering the imputation of Adam’s sin, and stripping out any importance to the organic connection of Adam as source and root of humanity in favor of a “strictly” federal union, how is this less problematic than simply saying that God created a sinful race?

The Bible, the history of Christian doctrine, and the Reformed heritage in particular has much more to offer to explain Adam’s and then Christ’s representational role.  Neglecting this will tend to leave those who do so with an arbitrary and nominalistic view of how God deals with us.