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I think the reason why is that whereas the Sacred Heart of Jesus was traditionally a symbol of combativity and militancy for Christendom, Divine Mercy represents happy, unconditional "mercy" towards sinners (without justice). It explains the drastic changes in approach between pre & post Vatican II Catholicism. Sacred Heart of Jesus is militant in spirit and used in all counter-revolutionary movements since the time it was revealed to St. Margaret Mary back the 17th century; but the whole Divine Mercy devotion does not represent the Catholic spirit at all. "The Catholic spirit is one of making constant reparation in penance for our sins, of praying for the graces of God, for the mercy of God in this life".
TIA has a great article contrasting the two devotions here: http://www.traditioninaction.org/polemics/F_07_DM_01.htm
"A typical Divine Mercy image remindful of a whirling dervish"
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Notice that in this modern fantasy, there is nothing for you to see as far as having to look upon the wounded heart of our Lord which was the price for this great mercy unto men. No, all that we see is the great outpouring of mercy which demands nothing from us, not even the contemplation of Christ's suffering as the greatest of mercies. The conciliar religion is a one way handout from God to men, a type of divine welfare system.
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The hands of Our Lord in this Divine Mercy picture do not show the marks of the nails, which therefore does not offend Protestants. Some of these pictures go as low as His feet, but they show no nail marks there, either. Similarly, since Protestants have complained about the "disembodied heart" featured in Sacred Heart images, that too is missing. The Newchurch approach is to lower Catholic standards so as to become acceptable to Protestants, whom they call "non-Catholic Christians" and "our brothers in the faith."
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It seems to me these points are important to keep in mind when thinking of "the Divine Mercy." I have heard Newchurch faithful praying the Rosary, when at the Decade Prayer they say,
"O my Jesus, forgive us our sins, save us from the fires of hell. Lead all souls to heaven especially those in most need of The Divine Mercy." So they drag it into the Rosary as if they were using their rosary beads to recite the Divine Mercy Chaplet.
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Since this image portrays this Divine Mercy pouring out like rain, it is not in conflict with universal salvation and syncretism.
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The foundation of this phenomenon is firmly ensconced in Vatican II, for in the Opening Speech of John XXIII he said the Council would not condemn error, and that instead of condemnation (of error) the council will seek to apply the "medicine of mercy."
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The problem with that is, of course, that mercy is not medicine, but penance
is medicine. All this was in the shadow of Fatima where Our Lady emphasized
the medicine of penance to the 3 children. And John XXIII chose to ignore Fatima.
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