You will only begin to truly understand the moral teaching of the Bible when you become like its Author, when he dwells in you, and you in him. The risen Christ spent the time before his Ascension opening the Scriptures to his disciples. We tend to think that we can go to the Bible and find out what it says, and then obey it or not as we wish. It assumes that anyone can discern God's law as easily as he can observe the law of gravity. Modern liberalism or skepticism, born from the great advance in the scientific study of the Bible in the last century, began with this assumption.
Positivism fails in spiritual things because it leaves out the human heart.
The heart is the instrument through which we see and measure spiritual things. If your heart is corrupt and unredeemed, you won't be able to see with it-or rather more worrisomely, its subtle flaws may even cause you to see something that isn't there while thinking you see rightly. Even those who believe it allows for war or capital punishment know that it forbids the killing of innocent people. But in the Middle Ages certain corrupt popes and Christian princes decided that it didn't apply to Muslims in Palestine, and today certain ideologues in the mainline churches believe it doesn't apply to unborn children. The popes' interpretation of the commandment was distorted by their lust for power and control, and I suspect that the ideologues' interpretation is often distorted for the same reason.