One of the many promises that Jesus gave to his apostles was that after his departure from them the Holy Spirit would come to them (John 14:16-18). In what way would this happen?
Jesus said that the Father would send the Spirit in response to a future prayer by Jesus. I suppose we could ask why Jesus did not make that request at the time when he was speaking to the disciples. The answer to such a question is that the petition would follow the exaltation of Jesus.
The Saviour also indicated that the significance of the presence of the Spirit would be the presence of Jesus with them even although he would be exalted in heaven. In a way we cannot understand the physical absence of Jesus because of his ascension would not mean that Jesus was no longer with them.
Yet we know that things were different for Jesus after his ascension. When he entered heaven at that time he did so as the Mediator about to be exalted by his Father. And one of his roles as Mediator is to extend the kingdom of God and also to engage in building the church for which he died. He would do this through the ongoing activity of the Holy Spirit.
In an obvious way, Jesus kept this promise when he sent the Promise of the Father (the Holy Spirit) on the Day of Pentecost. Should we describe it as the presence of the Spirit or the presence of Jesus? Both, as long as we remember that Jesus is exalted in heaven.
This promise by Jesus should have encouraged the disciples in the Upper Room. It does not seem that they were listening very well. Even so, it was a promise from Jesus that he would make them effective servants in the work he had given to them to fulfil.
This promise gives us insight into the priestly ministry of Jesus in heaven. Perhaps we could take a few minutes to read Psalm 67, a song that describes the fulfilling among the nations of the Abrahamic covenant. As Paul reminds us in Galatians 3:14, the coming of the Spirit is a fulfilment of that covenant. And Jesus, the priest on the divine throne, engages in constant conversation with the Father as to how and when the Spirit will next further the growth of the gospel. The coming of the Spirit was not just for the days of the apostles.