The image of the Sacred Heart on fire is directly from the private revelations of St. Margaret Mary Alacoque, but is also very biblical.
Many depictions of the Sacred Heart show flames coming from the top of Jesus’ heart. This type of symbolism was not used in the Church until the 17th century, when St. Margaret Mary Alacoque had a private revelation of Jesus.
She wrote down the words Jesus told her and he specifically mentions “flames” coming from his heart.
My Divine Heart is so inflamed with love for men, and for you in particular that, being unable any longer to contain within Itself the flames of Its burning Charity, It must spread them abroad by your means, and manifest Itself to them (mankind) in order to enrich them with the precious graces of sanctification and salvation necessary to withdraw them from the abyss of perdition.”
The idea of God being associated with a “fire” is also very biblical. God himself revealed his “name” to Moses in the midst of a “burning bush.”
There the angel of the Lord appeared to him as fire flaming out of a bush. When he looked, although the bush was on fire, it was not being consumed.Exodus 3:2
God also “preceded them, in the daytime by means of a column of cloud to show them the way, and at night by means of a column of fire to give them light.” (Exodus 13:21)
Jesus used fire to communicate his desire to spread his love to all humanity.
I have come to set the earth on fire, and how I wish it were already blazing!Luke 12:49
Even the Holy Spirit appeared at Pentecost as “tongues of fire” (Acts 2:3).
While it may seem strange to see the Sacred Heart of Jesus on fire, it is a symbol that has biblical connections in addition to its origin from the private revelations of St. Margaret Mary Alacoque.