Below are my reflections on the meaning of the symbolism of the Sacred Heart of Jesus image.
The Heart itself.
The heart is a universal symbol for love. "If anyone acknowledges that Jesus is the Son of God, God lives in him and he in God. And so we know and rely on the love God has for us. God is love. Whoever lives in love lives in God, and God in him" (1 Jn. 4:15-16). God so loved world he gave His only Son as a sacrifice for our sins.
The heart as an organ pumps blood to every member of the body. So too the Sacred Heart of Jesus pumps His blood to each one of us especially when we receive Him in the Eucharist in a state of grace. The heart is life giving. One man died that we all might have life. This is the heart which literally poured out its blood and water on our behalf to give us eternal life.
The crown of thorns.
The crown of thorns around the head of Jesus was placed there by the soldiers as they struck him and mocked him saying, "All hail the King of the Jews." I don't have the source of this next quote, but Jesus is supposed to have said to Margaret Mary Alacoque that the soldiers gave him a crown of thorns on his head and his friends gave him the crown of thorn around his heart. The love of men has grown cold in the last days and the heart which has love all has not been loved in return. It has been hated, reviled, or just ignored.
The crown of thorns surrounding the Sacred Heart is also like a hedge of protection.
Satan asked God about Job, "Have you not put a hedge around him and his household and everything he has" (Job 1:10)? In the book of Hosea, God put a spiritual hedge of thorns around Gomer. "Therefore, I will block her path with thorn bushes; I will wall her in so that she cannot find her way. She shall chase after her lovers but not catch them; she will look for them but not find them. Then she will say, "I will go back to my husband as at first, for then I was better off than now" (Hosea 2:6-7). There is also an opinion in Jewish rabbinical tradition that the bush from which God spoke to Moses was a thorn bush. (See the following article from the Jewish Encyclopedia.) The Law on Sinai was given forth from a thorn bush; the Law on Calvary was given forth from the heart of a man pierced with nails, spear, and thorns. He spoke the words earlier in John's Gospel, "I give you a new command: Love one another as I have loved you." He lived those words unto his death on the cross and his rising from the dead.
When we are in the Sacred Heart of the God who is love and He within us, we are protected from the fiery darts of the evil one. God allowed Satan to try Job with the loss of all of his possessions and family, even the loss of his health, but God did not allow Satan to take his life. "God is faithful; he will not let you be tempted beyond what you can bear. But when you are tempted, he will also provide a way out so that you can stand up under it (1 Cor. 10:13).
The fire in the Sacred Heart is the fire of Divine Love. The fire from which God spoke to Moses burned but did not consume the thorn bush (Ex. 3:2). John the Baptist said of Christ, "His winnowing fork is in his hand, and he will clear his threshing floor, gathering his wheat into the barn and burning up the chaff with unquenchable fire" (Mt. 3:12). Again Paul writes:
"For no one can lay any foundation other than the one already laid, which is Jesus Christ. If any man builds on this foundation using gold, silver, costly stones, wood, hay or straw, his work will be shown for what it is, because the Day will bring it to light. It will be revealed with fire, and the fire will test the quality of each man's work. If what he has built survives, he will receive his reward. If it is burned up, he will suffer loss; he himself will be saved, but only as one escaping through the flames" (1 Cor. 3:10-15).
Those who love Jesus are in his Sacred Heart and they are purified by him like gold that is tested in fire. Though we suffer loss of imperfect work built from inferior materials, we shall be saved in the end. I think that purgatory is within the Sacred Heart of Jesus figuratively and literally. Though the heart is a symbol, the love it symbolises is quite literal. Those who do not love Jesus will suffer the eternal loss of God's friendship. There is a consuming fire that goes before the Israelites and destroys their enemies. "For our God is a consuming fire" (Deut. 4:24; Heb. 12:29). Those who die without the fire of God's love in them are eternally punished by it.
The cross in the middle of the flames in his Sacred Heart is the suffering which we endure just as he endured suffering. "If anyone would come after me, he must deny himself and take up his cross daily and follow me (Lk. 9:23). He gave his life on the cross that we might live. The greatest good of our salvation was brought about through the greatest evil of deicide. "And we know that in all things God works for the good of those who love him, who have been called according to his purpose" (Rom. 8:28).
If we are within his heart, though we suffer the cross with him, and we are burned by the fire of our trials in life, we are not consumed by them. We are protected as the three young men were protected in the furnace by the angel in the third chapter of the book of Daniel. We are purified as gold is tested in fire. We are safe and protected within the womb of his heart. We cling to the promise he gave that we will not be tempted beyond our endurance. He is always faithful to his promises.
There are 12 promises that he gave to St. Margaret Mary Alacoque listed below:
1. I will give them all the graces necessary for their state of life.
2. I will give peace in their families.
3. I will console them in all their troubles.
4. I will be their refuge in life and especially in death.
5. I will abundantly bless all their undertakings.
6. Sinners shall find in my Heart the source and infinite ocean of mercy.
7. Tepid souls shall become fervent.
8. Fervent souls shall rise speedily to great perfection.
9. I will bless those places wherein the image of My Sacred Heart shall be exposed and venerated.
10. I will give to priests the power to touch the most hardened hearts.
11. Persons who propagate this devotion shall have their names eternally written in my Heart.
12. In the excess of the mercy of my Heart, I promise you that my all powerful love will grant to all those who will receive Communion on the First Fridays, for nine consecutive months, the grace of final repentance: they will not die in my displeasure, nor without receiving the sacraments; and my Heart will be their secure refuge in that last hour.
Sacred Heart of Jesus have mercy on us!