This passage predicts that the Gentiles will turn from the Lord. If they don't repent the following promise is given:
'And I say unto you, that if the Gentiles do not repent after the blessing which they shall receive, after they have scattered my people--
Then shall ye, who are a remnant of the house of Jacob, go forth among them; and ye shall be in the midst of them who shall be many; and ye shall be among them as a lion among the beasts of the forest, and as a young lion among the flocks of sheep, who, if he goeth through both treadeth down and teareth in pieces, and none can deliver.
Thy hand shall be lifted up upon thine adversaries, and all thine enemies shall be cut off' (3 Nephi 20:15-17).
Joseph Smith
"Thus after this chosen family (the house of Israel) had rejected Christ and His proposals, the heralds of salvation said to them, 'Lo we turn unto the Gentiles;' and the Gentiles received the covenant, and were grafted in from whence the chosen family were broken off: but the Gentiles have not continued in the goodness of God, but have departed from the faith that was once delivered to the Saints, and have broken the covenant in which their fathers were established (see Isaiah 24:5); and have become high-minded, and have not feared; therefore, but few of them will be gathered with the chosen family. Have not the pride, high-mindedness, and unbelief of the Gentiles, provoked the Holy One of Israel to withdraw His Holy Spirit from them, and send forth His judgments to scourge them for their wickedness? This is certainly the case." (Teachings of the Prophet Joseph Smith, p. 15)