3 Nephi 23

Having taught the words of Isaiah to the people of the Americas, in a sense reviewing that which they already had, He counseled and then commanded -

"...ye ought to search these things. Yea, a commandment I give unto you that ye search these things diligently; for great are the words of Isaiah."  (Book of Mormon 3 Nephi 23:1)

And then He explains why -

"For surely he spake as touching all things concerning my people which are of the house of Israel; therefore it must needs be that he must speak also to the Gentiles.
3 And all things that he spake have been and shall be, even according to the words which he spake.  (Book of Mormon 3 Nephi 23:2 - 3)

As He speaks of the "Gentiles", in my mind this would include those that are descendents of the house as well as those that are not which I believe are few.  I believe that because these people probably considered all that were not "Jews" were gentiles.  That is why it is so important to study these words because the words of Isaiah apply to all.

"Therefore give heed to my words; write the things which I have told you; and according to the time and the will of the Father they shall go forth unto the Gentiles.
5 And whosoever will hearken unto my words and repenteth and is baptized, the same shall be saved. Search the prophets, for many there be that testify of these things."  (Book of Mormon 3 Nephi 23:4 - 5)

This final command then includes searching not only the words of Isaiah which are great, but the words of all the prophets, even modern day prophets, because they testify of these truths.