Jesus is our example in all things, and he taught from the scriptures. Here he particularly commends the words of Isaiah to us; in fact, he commands us to search them. Note the sequence of the wording: He tells us it is our duty to search the words of Isaiah, then he adds that it is a commandment. We are not just to read or even study but to search them and not just passively but diligently. Seldom does the Savior single out a certain prophet and use adjectives of praise for his writings, but here he eulogizes the contributions of this one prophet of ancient Israel: “great are the words of Isaiah.”
Why we should search the writings of Isaiah and how to understand them is explained in “Why Study Isaiah” and “How to Understand Isaiah,” found just before the commentary at 1 Nephi 20.