What is Biblical morality? This is an important question, because if we can figure out exactly what it is (or as close as is humanly possible), then we can better adapt to new situations, as well as being able to better show why it should be followed.
In one sense, Biblical morality can be summed up as "that which God commands us to do (or not do)". This may seem like an overly simplistic answer, but it's actually quite literally true. Right and wrong is determined entirely by God's decree. We can see this stated in Romans 5, where Paul says "Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned— for sin indeed was in the world before the law was given, but sin is not counted where there is no law. Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come." In particular, look at verse 13, where he says that "sin is not counted where there is no law." What he's saying here is that, without the law to specify what is right and wrong, then nothing would be right or wrong! This perspective on morality is known as objectivism: the belief that right and wrong are determined by a power beyond human control or preference.
I said earlier that that is Biblical morality "in one sense." It's entirely true, but there is more to it. You see, God is logical. He doesn't give commands arbitrarily, for the sake of having commands. To see this, you only need to look at the rest of His creation. Everything has a purpose and a function. Humans are composed of many parts, each of which serves a purpose as part of the whole. (Even some parts that used to be thought of as "junk.") This is seen in every intricate working of the universe, and it must be the same for morality. So, knowing that right and wrong are determined because God decrees it, the next question is: why does He decree these particular commands? Or, to put it another way: why is Biblical morality?
At this point, it helps to look at the classic categories theologians have set the law into (note that these are defined by humans, and thus fallible - not necessarily mistaken, though!) The law is generally divided into Civil, Ceremonial, and Moral.
The civil law was basically the law of Israel used in the common sense of the word. It was equivalent to the laws given by a government, and it was given because Israel had no government of their own. Most people will agree that we don't have to hold to the civil law today, though it, like every other part of the Bible, is worth studying and considering.
The ceremonial law served an incredibly important purpose, and yet is also not necessary to follow today, for a very different reason. What it did was foreshadow the coming of Jesus. An excellent book on this can be found in John Sittema's Meeting Jesus at the Feast, which talks about how the feasts and festivals which were ordained in the Old Testament showed Jesus' coming. Since the meaning of this law has been fulfilled, we are no longer required to follow it. This is especially important to know, because people attacking Biblical morality will often use ceremonial laws as examples of how Christians will "cherry-pick" the commands that suit them, by demonstrating our failure to follow seemingly arbitrary or ridiculous commands such as not eating certain foods, or not wearing clothes containing more than one type of fabric. Acts 10 is one of the clearest illustrations of the meaning behind these sorts of commandments being fulfilled.
Finally, there is the moral law. This, too, must have a purpose, but that may be a more difficult question to answer. There are two things which must be considered here: why have a moral law at all, and why these particular commandments?
The first question may seem odd. Why ask why we should have a moral law? To answer this, we need to go back to the previous passage from Romans 5. Looking again at verse 13, we see something interesting. If "sin is not counted where there is no law," then why have a law at all? Without a moral law to break, wouldn't humans be perfect, and all deserving of heaven?
The previous verse, however, seems to disagree. Paul tells us that even without a law, there was still sin in the world. However, it was "not counted." This tells us that sin is something more than disobeying commands, something less tangible. But doesn't this completely change our definition of Biblical morality, if sin is not simply disobeying God's commands? Well, no. But it does necessitate some refinement. Instead of our previous definition, we can say that right and wrong are "obedience to or rebellion against God."
To get an idea of what I mean by this, we should look at Romans 6 (the whole chapter). In this chapter, Paul talks about being slaves to God or sin, right or wrong. He connects sinful acts with slavery to sin, and vice versa, with slavery to God being connected to following His commands. From this, we can see that one leads to another: a sinful nature causes us to sin.
As an example of the distinction between sinful nature and sinful acts, think of a child who has been told to do the dishes by his mother. Though he may do as he has been told, he may still deeply resent his job, and even hate his mother for it. Though his actions do not go against the "law" laid down be his mother, his nature is still rebellious, and if it shows through he may even be punished for it. So there can clearly be a distinction between the nature and the actions, though they are still tied together: because of his rebellious nature, he may refuse to do the dishes, but without it, he would happily comply.
So while the law may make active rebellion possible, passive rebellion can exist either way. Even if God gave no commands for us to disobey, we would still be slaves to our sinful nature. And it is our sinful nature that condemns us: it is clear throughout the Bible that sins themselves can be forgiven, but salvation is only attained through embracing Jesus as our savior, and putting behind our "old selves." Disobedience to the law is an outward manifestation of our corruption, yet it is not our corruption itself.
Rather than explaining why we should have a law at all, I have explained why having a law is not a negative thing - which it may seem to be, from that verse in Romans 5. The question of why it is a good thing to have a law, then, can be answered in the next question: why this law? To understand why we are commanded, we must understand the commands.
This question is much simpler to answer: because these commands are good. By "good," in this case, I mean "beneficial." I noted before that God is logical, and so it makes sense that the things He tells us to do are those which are best for us. Consider a few of the 10 Commandments: "do not steal," "do not murder," "do not bear false witness" - these are not just morally right, they are practical guidelines for society. Jesus said it best when he summed up the commandments as "love the Lord your God with all your heart and with all your soul and with all your strength and with all your mind, and your neighbor as yourself." What better basis for morality could we have?
So in summary, Biblical morality is that what God decrees of us, and He decrees it because it is best for us. Sometimes that may seem difficult to remember, because the temptation to sin is strong. But when you think about His law for us, it only goes to further show His grace.
(Many thanks to my friend Cassie for helping me edit this post! By which I mean completely rewrite it.)