6 “And the congregation of the people shall surround You.” This may be understood two ways. For the congregation of the people can be taken, either of them that believe, or of them that persecute, both of which took place in the same humiliation of our Lord: in contempt of which the multitude of them that persecute surrounded Him; concerning which it is said, “Why have the heathen raged, and the people meditated vain things?” But of them that believe through His humiliation the multitude so surrounded Him, that it could be said with the greatest truth, “blindness in part is happened unto Israel, that the fullness of the Gentiles might come in:” and again, “Ask of me, and I will give You the Gentiles for Your inheritance, and the boundaries of the earth for Your possession.” “And for their sakes return Thou on high:” that is, for the sake of this congregation return Thou on high: which He is understood to have done by His resurrection and ascension into heaven.
For being thus glorified He gave the Holy Ghost, which before His exaltation could not be given, as it is written in the Gospel, “for the Holy Ghost was not yet given, because that Jesus was not yet glorified.” Having then returned on high for the sake of the congregation of the people, He sent the Holy Ghost: by whom the preachers of the Gospel being filled, filled the whole world with Churches.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)