Hear, O Israel: The LORD our God is one LORD; and you shall love the LORD your God with all your heart, and with all your soul, and with all your might.
This verse, along with "You shall love your neighbor as yourself" is the Law and the Prophets. Do this and the rest of the entire Old Testament is just window dressing. Of course, the fact that none of us does this is why the rest of the Old Testament exists. It's not enough to know what you have to do. After that, you have to actually do it, and there's the rub. It's because none of us do this commandment that God sent the rest of the Law and the Prophets exist. And they exist, not merely to give us more good advice to ignore, but to point us to Jesus Christ who alone is able to make us capable of doing the Law and the Prophets.