If we look at the 10 Commandments, (which were simply a condensation of the entire Law of the Old Testament) and the Beatitudes, they seem at first to be two different things. However when we look at them more closely, we see that they express the same truths, but from an opposite perspective.
The Decalog, tells us what we should Not do. The Beatitudes encourage us in what we Should do. As both are found in Scripture, and both were taught to us from God Himself, they are both important to our spiritual lives.