Pentecost is the feast of the Spirit. “Tongues of fire” is a figurative way of explaining that the Holy Spirit descended upon the apostles. Before Pentecost, the apostles were huddled in an upper room praying the first novena for the coming of the Holy Spirit. Jesus had promised that ten days after His Ascension He would send the Third Person of the Blessed Trinity, the Holy Spirit. The apostles were quite scared and remained in prayer for nine days with the Blessed Mother. After completion of the prayer, the Spirit came and enlivened the twelve apostles.
In depicting this event, the Holy Spirit is pictured either as a dove or as a flame of fire, fire because the result of the gifts of the Spirit is motivation for the Christian. The old axiom about “lighting a fire under someone” as motivation is analogous to the effects of the Spirit. The Seven Gifts of the Holy Spirit bear twelve fruits and ultimately motivate the person to be “on fire with the love of God.” Therefore, the image evoked by the tongues of fire is one of enthusiastic faith. After the reception of the Holy Spirit, the apostles could expel demons, talk in many languages, and endure torture even to the point of martyrdom.