What is the point of citing Bible verses when making moral arguments? Since not everyone believes the Bible to be true or authoritative, surely we need to make arguments based on something else, something we have in common. But Christians use the Bible all the time. Why?
To get to an answer, one has to consider a range of issues in ethics. Once one has reasoned through these questions, it becomes clear why many Christians find that the Bible has an essential role to play in most moral reasoning. Thus, when engaging in moral debate, we often use the Bible.
The first relevant issue is whether moral statements are translatable into factual statements. For example, can one translate "it is wrong to murder" into a factual statement? Some suggest that this is not possible, that concepts such as 'good' and 'wrong' are not analyzable in terms of anything else. Christians, however, are likely to say that "it is wrong to murder" can be translated into the fact that God commands us not to murder. If this is true, then the source of moral norms must be God. This means that Christians will say that the source of moral norms are prescriptions of the will of God rather than merely the deliverances of reason or the situation one finds oneself in.
The Christian then sets out to establish some kind of criteria for what actions count as good or bad. There are a couple of ways to achieve this. The first supposes that what makes an action good or bad is the consequence of an action. The second claims that the criteria are found within the commands of God. The latter makes sense because the Christian is already committed to thinking of the source of moral norms as the commands of God. Thus, most Christians will hold that the criteria for the mortal status of a particular action is whether it is in accordance with God's commands.
The second issue is whether we can justify the factual claim. Does God, in fact, command us not to murder? The question of justification is not merely one the Christian has to answer. All ethical systems must say why it is that they are right and others are wrong. According to the relativist, there is no way to justify a moral claim. However, the relativist will have to show us why we should think that there are no moral norms that are not identical to cultural norms. According to non-cognitivists, moral claims are not facts but merely expressions of desires. Again, the non-cognitivist will also have to show that there are no objective moral norms and only desires. According to many people, moral norms are objective but are known primarily by intuition. They might say that "it is wrong to murder" is self-evidently true. Consequently, it has no need for justification. The problem is that there are many moral norms that lots of people don't find to be self-evidently true.
So, the Christian justifies her moral claims by showing that God has prescribed or forbidden certain actions. For example, God has commanded us not to murder one another. However, it is not enough to show that God has given a command. One must also try to show why he has done so. This is also a matter of presenting what the Bible says. "It is wrong to murder" can be translated into the fact, "God commands us not to murder" and the reason he does so is that he has made us in his image thus bestowing the kind of intrinsic value onto us that would make it wrong for us to murder each other.
Of course, one might merely say "Christianity is false, so why should I accept the Bible's prescriptions for a moral life?" This leads to another (and separate) question: Are the claims of the Bible true or false? However, the same burden falls upon all ethical systems. They are all based in certain views of the world. Thus, for all moral claims, one must engage in justifying the worldview that produces them.
Christians do also believe that there is a certain sense in which everyone knows--to some degree--the moral law of God. The Bible tells us that the moral law is written on human hearts (Romans 2:15). Thus, all human beings can believe some of God's laws without looking in the Bible. Those who do not believe the Bible also have consciences that accuse them of breaking some of those commands. Many people who perform a sinful action feel bad about it even when they can't think of a reason for why the action is wrong. This is the work of the conscience that God has given human beings.
However, to justify those moral norms, one can't merely appeal to an inner belief. One must also say why that inner belief is the right one. And for that one needs the Bible. This is why we use the Bible in much of our moral reasoning.
Additional Note: Some forms of moral reasoning do not require the use of the Bible. Instead, these forms of reasoning ask people to be consistent within their own moral system. For example, if one thinks that all moral norms are culturally determined, then one cannot also call out other cultures for their actions on any moral grounds.
The above is found in more detain in John Feinberg, Ethics For A Brave New World, 21-40; 49-52.