IV. If the Divine mercy is to be exercised, the heretics must cease entirely from the error of their ways.
What therefore the unholy parricide has perpetrated by seizing on the holy Church and cruelly murdering its very ruler, cannot be expiated by man's forgiveness, unless He Who alone can rightly punish such things, and alone can of His unspeakable mercy remit them, be propitiated. But though we are not anxious for vengeance, we cannot in any way be allied with the devil's servants. Yet if we learn they are quitting the ranks of heresy, repenting them of their error and turning from the weapons of discord to the lamentations of sorrow, we also can intercede for them, lest they perish for ever, thus following the example of the Lord's loving-kindness, who, when nailed to the wood of the cross prayed for His persecutors, “Father, forgive them; for they know not what they do.”
And that Christian love may do this profitably for its enemies, wicked heretics must cease to harass God's ever religious and ever devout Church; they must not dare to disturb the souls of the simple by their falsehoods, to the end that, where in all former times the purest faith has flourished, the teaching of the Gospel and of the Apostles may now also have free course; because we also imitating, so far as we can, the Divine mercy desire no one to be punished by justice, but all to be released by mercy.
Source: Letters (New Advent)