Saint Gertrude (d. 1302) had a vision on the feast of John the Evangelist. In it, she rested her head near the wound in the Jesus’ side and heard the beating of His Divine Heart.
She asked Saint John why he had never spoken of Jesus’ Heart beating (he t0o had rested on it during the Last Supper) and the Saint replied that this revelation had been reserved for when the world had grown cold and would need of it to rekindle its love.
Private devotion to the Heart of Jesus began to spread, but it wasn’t until 350 years later that Jesus appeared to St. Margaret Mary Alacoque and devotion spread worldwide.
The Sacred Heart is the chosen image of Jesus’ Divine Love that he has given us. Surmounted by a cross, wreathed with the crown of thorns, and pierced, we clearly see that the Passion wounded the very heart of God. And Jesus underwent the Crucifixion to save us – it is His ultimate act of love.
This is particularly apparent in this rendition with the anatomical heart.
As for the fire; the heart is the center of emotions. We often say our heart burns when we long for something very deeply – and Jesus longs for us more deeply than we can imagine.
Fire also gives off light, and Jesus is the Light of the World. It consumes the burnt offerings in the Old Covenant. And it is a symbol of the Divine Incarnate – such as the burning bush in Exodus.