One of the hallmarks of the Abrahamic religions, of which Judaism, Christianity, and Islam are the most significant subsets, is the claim to moral objectivism that was originally made by each of these religions. By "moral objectivism," we are referring to a position in moral philosophy that holds that certain moral statements are universally true or universally false at a given time. By definition, a statement possesses the quality of objectivity if and only if that statement possesses a truth value that is independent of individual interpretations. Any given statement cannot be universally true or false unless the truth value of that same statement is completely independent of individual human interpretation. For example, the statement 1 + 1 = 2 is an objectively true statement, because it is true regardless of what anyone thinks. Even if all human beings were to suddenly vanish, the statement 1 + 1 = 2 would still be true, even if absolutely nobody in the universe actually knows that equation. Likewise, even if all human beings were somehow brainwashed into thinking that 1 + 1 = 3, it does not logically follow that 1 + 1 = 3 is actually true. Because this statement has an objective truth value, the opinions of human beings have absolutely no bearing whatsoever on the truth value of the statement.
The philosophical position of moral objectivism asserts that moral statements, like the mathematical statement 1 + 1 = 2, can also have objective truth value, meaning that they are either true or false regardless of what any individual human being thinks. For example, a moral objectivist might assert that the moral statement "One ought not kill other humans" is objectively true, meaning that killing other humans is universally wrong (immoral). If any person or any culture engages in, supports, or condones the killing of other humans, then that person or culture would be deemed to be objectively immoral in this moral objectivist framework.
As you can imagine, moral objectivism is an attractive position for many people, because it offers a feeling of moral certitude and a sense of natural law that transcends individual human beings. In the absence of objective morality, there can be no absolute moral certitude, and all systems of morality would have to be derived from subjective human opinion. Because human beings are fallible and imperfect beings, there is always the specter that our subjective moral systems are by extension fallible and imperfect. Such a concern naturally leads to insecurity and anxiety, sometimes on a massive scale (often associated with societal upheaval of some sort, which is often followed by the advent of some new religion or ideology or a call to return to old ways), and it seems to me that the desire to follow a perfect code of morality that is free of all human corruption is part of what made the Abrahamic religions in particular (which make claims to moral objectivism) so attractive to so many people throughout history. The Abrahamic religions offer a degree of certitude and solace that is unmatched by virtually any other ideology or belief system, leading to unparalleled psychological attachment among the followers of these religions.
In the Abrahamic tradition, what exactly is the source of morality, and what makes this morality objective? The answer to the first question is God Himself. It is God that promulgates moral statements for human beings to follow. The most famous and most explicit example of God promulgating morality to human beings can be seen in the revelation of the Ten Commandments - a collection of ten moral statements - from God to Moses. To answer the second question, the Ten Commandments possess the quality of objectivity because their ultimate source (God) is separate from all human beings. If God is separate from human beings, then it logically follows that the moral statements issued by God are themselves separate from human beings, such that certain moral statements would still retain their truth value even if all human beings disbelieved in God, or all human beings followed some other system of morality.
At face value, this argument makes sense (given, of course, the initial assumption that the Abrahamic God does in fact exist), and indeed presents a strong case for tying moral objectivism with religion (specifically the Abrahamic religions). However, there is a significant pitfall: God does not talk to us human beings directly, and the practical source of morality is not God Himself, but actually the written texts (and to be more precise, particular interpretations of written texts) which purport to contain His Word. If God Himself actually appeared before humanity and directly issued moral commands to us, in such a way that was empirically verifiable, then we could indeed say without hesitation that these moral commands pertain to objective moral truths that exist separately of any individual human being's interpretation, opinion, or imagination. However, this is not the case. What we have instead are written texts which are open to interpretation, and it is the interpretations of the written texts which ultimately guide our morality. The act of interpretation is, by its very nature, a subjective act, and as such, our sense of morality which we derive from our interpretation of a holy text (whatever that text may be) must also necessarily be subjective to some degree.
A simple example from current events can be used to illustrate this point. As many are no doubt aware, the group known as ISIS or Islamic State has carried out a spree of violent killings, rapes, and sexual enslavement throughout large swathes of the Middle East over the past two years. Are these acts merely the result of a depraved nihilism? Many would say "yes," but the group itself has clearly and adamantly maintained, from the very beginning, that its actions are in accordance with the Quran and the Sunnah (established traditions and sayings of Muhammad). Whenever ISIS carries out one of its major attacks, it almost always makes sure to provide scriptural justification. For example Quran 4:24 permits Muslim men to have sex with captured slaves ("those who your right hands possess"), which established a theological justification for ISIS to engage in sexual slavery. Likewise, Quran 9:29 urges Muslims to "fight against those who disbelieve" (i.e. infidels) until they are humbled and pay jizyah (religious head tax), and ISIS uses this verse to justify their attacks on disbelievers and monetary exactions. Now, it is perfectly possible for a Muslim to reject this interpretation and denounce ISIS as "false Muslims," but the relevant question is: what makes YOU and other "moderate Muslims" right, and what makes ISIS wrong? It is not sufficient to simply say that the vast majority of Muslims do not agree with with ISIS. Even if 99.9% of all Muslims strongly disapproved of ISIS, and only 0.01% of Muslims approve of ISIS, that by itself is not proof that ISIS is objectively immoral according to Islam.
In order to show that ISIS is objectively wrong, you have to show that ISIS is wrong regardless of what the prevalent opinion is - to argue otherwise is to commit the logically fallacy of argumentum ad populum. If 99.9% of people believe that the sun revolves around the earth, they are not correct simply by virtue of being in the overwhelming majority; the 0.01% who believe that the earth revolves around the sun are still correct, because they hold a position that is objectively true. Likewise, it is perfectly possible for ISIS to be more "correct" than other Muslims despite making up only a small percentage of the total Muslim population; the remaining "moderate" Muslims are not "better Muslims" than ISIS simply by virtue of being in the majority. In Islam, as with other Abrahamic religions, it is Allah who judges what is right and wrong (as well as Muhammad, who for all intents and purposes is a demigod in Islam), not the so-called "moderate Muslim." Because no moderate Muslim is identical to Allah or Muhammad, no moderate Muslim can say with certitude that ISIS is morally wrong within the framework of Islam. The best they can do is offer their own interpretations in contradistinction to those of ISIS, or cite other scholars (i.e. other interpretations) that critique ISIS, but they are unable to show that their particular interpretation is objectively correct while that of ISIS is objectively wrong. Thus, from this one example, we can see the inability of religion to provide a clear sense of objective morality, as the only practical morality is that of the subjective interpretation that we chose to accept, and there is no means of objectively determining which interpretation is "correct" in the absence of direct guidance from God Himself.
