Fr. Scott A. Haynes
It has been said that “the Holy Spirit leaves no footprints.” In other words, the work of the Holy Spirit is mysterious. The disciples of the Lord could not even begin to understand the Paschal Mystery—how Christ would defeat death on the Cross—until they beheld Christ accomplish the works of our Redemption. Nor did they fathom how, after Jesus’ triumphal Ascension into heaven, they would receive the Holy Spirit on Pentecost.
Our Lord, in His kindness, perceives the apprehension of His disciples, and so He comforts them, saying:
“Do not let your hearts be troubled. Trust in God, trust also in Me.” 
In seeking to console them, Jesus teaches them about the Holy Spirit. He seems to be telling them that the best way to prepare for His absence is to open their hearts to receive the Holy Spirit.
The Apostles were saddened at hearing Jesus say to them:
“I tell you the truth: it is expedient to you that I go: for if I go not, the Paraclete will not come to you; but if I go, I will send Him to you.”