Jesus instructs the disciples in this verse as He prepares them for His imminent arrest and execution. Here, He indicates that this is the Holy Spirit, which is available only to those who believe in Christ. When one professes faith in Christ, the Holy Spirit resides permanently in his or her heart.
In addition to the Lord's work for us, the Holy Spirit of Truth continues to work within us since the Cross and Resurrection. It is impossible for those who do not know the Holy Spirit to receive the Spirit of God. However, those who have placed their trust in Christ as Savior have access to the Holy Spirit. In other words, He has come to make our body His permanent residence, the Temple of God.
Just before Christ completed His work of redemption on Calvary's Cross, Christ promised His disciples that God the Holy Spirit would be within them. This promise was fulfilled at Pentecost, and from that day forward, all those who trust in Jesus Christ as their Savior have God, the Holy Spirit, with them forever.

Dear Heavenly Father, thank You for sending me Your Holy Spirit to live within me and be with me always. It is a blessing to know that He is with us and within us to lead and guide us. Please let me become a willing vessel for Him to use to spread Your gospel of grace to a world that does not yet know You or cannot receive the Spirit of truth until they believe in Jesus and receive the gift of eternal life. 
In Jesus's name I pray. Amen.