Jesus said, "He that believeth on me...out of his belly shall flow rivers of living water. (But this spake He of the Spirit, which they that believe on Him should receive)." By "belly" is signified the innermost part of the person, the true and authentic bedrock reality of our existence as beings created in God's image. This promise was fulfilled on the day of Pentecost, after Jesus had ascended in glory to His Father, when the Holy Spirit descended on the Apostles as tongues of fire. The disciples who were formerly cowering behind closed doors for fear of the Jews were on this day strengthened and confirmed in their faith and empowered to preach the Gospel unto the ends of the earth.
Pentecost, however, is not merely an historical event, the so-called "birthday" of the Church. The same Spirit which established the Church and sustains her through the ages comes and abides in our hearts through the mysteries of Baptism and Chrismation. But the rite of Baptism alone will not save us. Though we have all received the Gift of the Holy Spirit in the waters of Baptism, this gift can be easily squandered or even scorned. There is no guaranteed passport to heaven. The path to salvation requires an ongoing struggle till we take our final breath to acquire the Holy Spirit of God.
According to St. Seraphim of Sarov, the acquisition of the Holy Spirit is the whole purpose of our life, and we acquire the Spirit in the very same way we acquire money. As merchants acquire wealth through trading one thing for another, so it is in the spiritual life. "Give blood, receive the Holy Spirit", according to the Fathers of the Church. In the case of the martyrs, this saying is fulfilled literally, but in a sense we "give blood" whenever we strive against the desires of our fallen human nature to fulfill the commandments of Christ.
Sadly, however, most of us devote more time and energy to our material wealth and security than we do to the salvation of our eternal soul. Truly we are more concerned with our earthly treasures than we are in the balance of our heavenly bank account. Of course we cannot buy our way into heaven. When the day of reckoning arrives (as it surely will) and we come to stand naked before the dread judgment seat of Christ, all that matters is how diligent we have been in our acquisition of the Holy Spirit.
It is for this very reason that the Church begins and ends each day with the prayer to the Holy Spirit: "O heavenly King, the Comforter, the Spirit of Truth...come and abide in us..." May we all pray this prayer continually from the depths of our hearts, from our "bellies," while striving ever to acquire ever greater gifts of the Holy Spirit.