For the evangelist tells us, that this referred to the Spirit, which believers were to receive after that Christ should be ascended into heaven. Those scriptures, Isaiah 49:10, Isaiah 58:11 Malachi 14:8, seem, among others, to be referred to in this promise of our Saviour.
Related Commentaries of John 7:39
John 7:39
39 (But this spake he of the Spirit, which they that believe on him should receive: for the Holy Ghost was not yet given; because that Jesus was not yet glorified.)