Albert Barnes Commentary John 7:39

"But this spake he of the Spirit, which they that believed on him were to receive: for the Spirit was not yet [given]; because Jesus was not yet glorified." — John 7:39 (ASV)

Of the Spirit. This refers to the Holy Spirit, who would be sent down to accompany their preaching and to convert sinners.

For the Holy Ghost was not yet given. The Holy Spirit was not given in such full and large measures as would be after Jesus had ascended to heaven. Certain measures of the influences of the Spirit had always been given in the conversion and sanctification of the ancient saints and prophets; but that abundant and full effusion which the apostles were permitted to behold afterward had not yet been given. See Acts 2:1-12; Acts 10:44–45.

Jesus was not yet glorified. Jesus had not yet ascended to heaven—to the glory and honor that awaited him there. It was a part of the arrangement in the work of redemption that the influences of the Holy Spirit would descend chiefly after the death of Jesus, as that death was the procuring cause of this great blessing. Hence he said (John 16:7), It is expedient for you that I go away; for if I go not away the Comforter will not come unto you; but if I depart I will send him unto you. See also John 16:8-12; John 14:15–16, 26.

Compare Ephesians 4:8-11.