Catholics believe that through the reservation of the consecrated host in the tabernacles of each of our parish churches and various chapels and oratories, Jesus is truly present in every Church. Because Jesus, the second person of the Blessed Trinity, exists in unity with the Father and the Holy Spirit, we also believe that the entire Trinity is present. The Church also teaches that God is present in God's Word and in each baptized person. God's omnipresence is an attribute which is frequently cited and to which we cling.
For the Hebrew people of Jesus' time, God was present in the Temple of Jerusalem. If one wished to be in God's presence, one made a pilgrimage to the Temple. In addition to that, the Jewish people believed that it was only possible to pray when ten Jewish men were present. Even then, they did not believe was present in their midst unless they were in the Temple. Jesus himself teaches us that He is present whenever two or three are gathered in His name.
Taking these tenets of the Jewish faith into consideration, we can readily understand how traumatic it must have been for the Jewish population of Israel when in 70-76 A.D., the Roman army, under the leadership of Titus, destroyed the Temple. Without the Temple, the Jews had no place to go to be in the presence of God. Around that time, a synagogue worship service developed, changing the nature of the synagogue. However, at about the same time, Christians were expelled from the synagogues, another very traumatic experience.
So in the space of about forty years, Jewish Christians lost Jesus through his crucifixion, the Temple through the Roman occupation, and the synagogue through the decision made by the Jewish elders. I don't believe we can understand the emotional impact these events would have had on these people.
John's Gospel was written after these historical events. Through his writing, John makes an effort to help the Christian community to overcome the effects of this emotional upheaval. He does that through Jesus' promise to send the Advocate whom we have come to believe is the Holy Spirit, infused into each one of us through the sacrament of Baptism. Jesus promised us that he would not leave us orphans. The Holy Spirit, the Advocate (Paraclete, Comforter, Counselor) is the fulfillment of that promise. When we read today's Gospel passage, these historical facts make it all the more understandable. When the Advocate comes whom I will send you from the Father, the Spirit of truth who proceeds from the Father, he will testify to me. And you also testify, because you have been with me from the beginning. I have told you this so that you may not fall away. They will expel you from the synagogues; in fact, the hour is coming when everyone who kills you will think he is offering worship to God. They will do this because they have not known either the Father or me. I have told you this so that when their hour comes you may remember that I told you. (John 15:26b – 16:4a)