[And after that ye were blessed]: This phrase refers to verse 26. Jesus is sent to bless them .His presence is fulfillment of that promise, and therefore they are currently in the condition of verse 26. Thus “after that ye were blessed” shows that the time of the promise will be after this appearance of the Messiah. This is an important point because the text cited from 1 Nephi above (1 Nephi 19:10-16) might be interpreted to mean that the gathering was coterminous with this appearance fo the Messiah.
[then fulfilleth the Father the covenant]: This is what comes after. The fulfillment is of the covenant promise to Abraham. This promise is ultimately fulfilled in the gathering that gathers not only the lineal and scattered House of Israel, but all those who spiritually accept the Gospel and become adopted in to the House of Israel (3 Nephi 16:12-13). It is in this context of the adoption of the Gentiles into the House of Israel that we get the next phrase.
[unto the pouring out of the Holy Ghost through me upon the Gentiles]: The context of these verses indicates that this particular statement does not refer to the final gathering, but to the events that will lead up to the final gathering. As was already pointed out to the Nephites on the previous day (3 Nephi 16) the future history of the Gospel will include a time when there will be persecution of the House of Israel at the hands of the Gentiles. This is the phase of the future history that is now being presented.
[which blessing upon the Gentiles shall make them mighty above all, unto the scattering of my people, O house of Israel.]: The initial acceptance of the Gospel among the Gentiles leads to the blessing of the Gentiles. This picks up the theme of the ultimate reversals from the previous day’s sermon, where the Gospel was taken from the Jews and given to the Gentiles, and then the Gentiles persecuted the House of Israel. In the end, however, the Gospel will return to the House of Israel and then the Triumphant Messiah will lead the House of Israel as the wicked (Gentiles) are destroyed. That is the ultimate reversal. At this point, however, the description is only of the phase where the persecution and continued scattering of the House of Israel is described.