Ahem (preparing to launch into my (abbreviated) speil on ethics...)!

   A virtue (goodness) is understood to refer to a standard by which a
thing ought be held.  If the thing matches the standard (the correct standard,
otherwise it would not be virtue), then the thing is good, otherwise it is
bad.  As an example, we say that a car acts correctly when it will run, and
incorrectly when it will not.  The standard of the car _running_ then would
be a virtue of the car.

   Now, just as the virtue of the car requires the judge to hold the car up
to the standard, so human virtue also requires the judge to hold us up to
the standard.  So, two questions arise: what is the standard (and how can
we know it), and also, who (why it must be a "who" is below) is the judge.

   Presupposing the existence of the standard (as we do here), we might 
imagine that a great many people know this standard.  We must note, however,
that it must surely be a rational mind which knows the standard.  A standard
is simply a form, without regard for the existence of the form (see the
is-ought fallacy).  Forms, however, can only be understood by the intellect,
and realized by existence, so an intellect (human, or otherwise) is required.
Moreover, although the same form may be in many minds, we would be sure to
know that an omniscient mind MUST contain the form-- since the standard would
be something that may be known, and omniscience is understood as knowing all.

  As for the question of judge, since any intellect may hold the form, we might
think that any may judge.  However, not all intellects may hold all forms, and
not all may witness the thing being judged (and thus know particulars and have 
the potential to compare them).  From this we find the intellect is necessary, 
but not sufficient to be judge. However, we also find that an omnipotent and
omnipresent being would be sufficient to act as judge.

   So, from the above we find 

1) Whatever the standard is, God knows it (since by 'God' we refer to the
   omniscient creator...)

2) Although many may judge, only God can judge completely (since only He is
   the omniscient and omnipresent creator).  And so, if there is virtue,
   God is the judge of virtue.

   And so, that is why the Christian can rationally say that God must be
consulted and referenced in questions of ethics.