So this sovereign Spirit is the eternal expression of the mutual love of the Father and the Son, the perfect fruition of Their love. He is the close embrace which unities Them eternally, the ineffable kiss of love which They eternally give to each other. "The Savior communicated the Spirit to His disciples," says St. Bernard, "in the form of a breathing, which was equivalent to His kiss, so that we might understand that the Spirit comes from the Father and the Son as a true mutual kiss."
This, then, is the most sweet kiss of His mouth which the enamored soul seeks with such ardor from the divine Spouse. The soul is lovingly united with Him through that ineffable communication of His Spirit in which are contained all the wonders of the charity of God.
From this notion springs the ancient custom of the Church of giving the faithful the Pax Dei in the form of the kiss of peace, which represents the mutual communication of the Holy Ghost. It is the Holy Ghost who will perfect our unity as members of that Church in the likeness of the divine Persons.
The Holy Ghost, as Spirit of Love, is symbolized by the dove, the emblem of faithful, pure, simple, and fruitful love. The Spouse calls the holy soul "My dove" because He sees that it is radiant and filled with the love of the Spirit. This is that affection for God spoken of by St. Irenaeus which leads us to the Father by means of the Word. Through the Holy Ghost we ascend to the Son, and through the Son to the Father. The Spirit is that love as strong as death, whose lamps are lamps of divine fire and flames. How vividly those souls feel this who are inflamed by the Spirit!" pg. 173-175 "The Mystical Evolution in the Development and Vitality of the Church" Volume 1 by Fr. John G. Arintero, O.P.