The standard of ethical and rational in the Christian worldview is God. To make ethical judgments on God, one has to propose a standard greater than God, that God would be subject to.
For those unfamiliar with the dilemma, it stems from Plato's dialogue between Socrates and Euthyphro, and was later rehashed by Bertrand Russell to refer to the Christian God.
It goes like this:
Socrates: And what do you say of piety, Euthyphro? Is not piety, according to your definition, loved by all the gods?
Socrates: Because it is pious or holy, or for some other reason?
Euthyphro: No, that is the reason.
Socrates: It is loved because it is holy, not holy because it is loved?
In terms of non-believers assertions then, is something good because God says so, or is God good because He adheres to a morally good code of conduct?
At first, almost all Christians will say something is good because God says so, and be done with it. God is sovereign and His commands must be obeyed. However, the further objection will be raised to say that "good" is then completely arbitrary. And back to our non-believer, if some Christians interpret that it is God's will to murder, then it becomes moral by God's whim, and is so justified.
If we then turn around and state that God is Himself subject to a universal moral code, that has obvious implications for God's omnipotence. Where does that code come from, and who or what decides what should be in there?
I just want to interject here, and point out that the dilemma applies equally well to any alternatives too. For example, if we are to assume that the majority of society determines right from wrong, then the question becomes: Is something good because society follows it, or does society follow it because it is good?
The dilemma for the Christian is not addressed by merely pointing out that the dilemma applies equally to all positions. The dilemma, as stated, leaves a no-win position for the Christian. Either good is arbitrary, or good is something that dictates to God.
In addressing this, as always, we have to look at what premises underlie the argument. We have to look at the logical foundation of morality, on what basis does it rest? To raise the objection that Socrates and Russell (and our objector here) did, they must first establish what it is what "good" means, how they logically came to know it, how they can logically apply it, by what authority does it demand compliance, and why is there an obligation to do good? It is here where non-believers have their feet firmly in mid-air.
The Christian answer is that this is a false dilemma. There are not only two options, there are three. There is an objective standard, which is internal to God. If there is a standard, then it is not arbitrary, and if it is internal to God, then God is not subject to an external authority. Morality is rooted in God's character. Whatever a good God commands, will be good.
There are further objections to this line of reasoning. If God is good, and good is God, it becomes a tautology, claims the objector, and tautologies are useless to explain things. However, once again the assumption is false. To say that God is good is to explain more about His character, it is not to conflate God and goodness. It explains more about the qualities of God, it does not state that God and "good" have the exact same identity.
To close then, what is "good" from a Christian perspective?
God reveals (ESV):
Gen 18:25 Far be it from you to do such a thing, to put the righteous to death with the wicked, so that the righteous fare as the wicked! Far be that from you! Shall not the Judge of all the earth do what is just?"
Job 34:10 "Therefore, hear me, you men of understanding: far be it from God that he should do wickedness, and from the Almighty that he should do wrong.
Job 34:17 Shall one who hates justice govern? Will you condemn him who is righteous and mighty,
Rom 3:5 But if our unrighteousness serves to show the righteousness of God, what shall we say? That God is unrighteous to inflict wrath on us? (I speak in a human way.)
Rom 3:6 By no means! For then how could God judge the world?
So how did Abraham, Job and Paul know that a just God does right, and not wrong, that He judges the wicked but not the good, and God does not do what is wicked?
Everyone recognizes what is right and wrong, even non-believers. The question is how does one make sense of that?
The answer is that when humans were made in the image of God, they were given moral intuition ability, and by falling to temptation, mankind acquired knowledge of good and evil to go along with the moral intuition. God reveals to us, no, He hard-wires into us what good and evil is.
Rom 1:17 For in it the righteousness of God is revealed from faith for faith, as it is written, "The righteous shall live by faith."
Rom 1:18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth.
Rom 1:19 For what can be known about God is plain to them, because God has shown it to them.
By faith in God can we make sense of God's righteousness, He has revealed it to us.