And what is the sum of purity? A heart full of mercy unto the whole created nature.
And what is perfection? Depth of humility, namely giving up all visible and invisible things….
Another time the same father was asked: What is repentance? He answered: A broken heart.
And what is humility? He replied: Embracing a voluntary mortification regarding all things.
And what is a merciful heart? He replied:
The burning of the heart unto the whole creation, man, fowls and beasts, demons and whatever exists so that by the recollection and the sight of them the eyes shed tears on account of the force of mercy which moves the heart by great compassion.
Then the heart becomes weak, and it is not able to bear hearing or examining injury or any insignificant suffering of anything in the creation.
And therefore even in behalf of the irrational beings and the enemies of truth and even in behalf of those who do harm to it, at all times he offers prayers with tears that they may be guarded and strengthened; even in behalf of the kinds of reptiles, on account of his great compassion which is poured out in his heart without measure, after the example of God.
[...] The sum of all is God, the Lord of all, who from love of His creatures, has delivered His Son to death on the cross. For God so loved the world, that He gave his only begotten Son for it.
Not that He was not able to save us in another way, but in this way it was possible to show us His abundant love abundantly, namely by bringing us near to Him by the death of His son.
If He had anything more clear to Him, He would have given it us, in order that by it our race might be His.
And out of His great love He did not even choose to urge our freedom by compulsion, though He was able to do so. But His aim was, that we should come near to Him by the love of our mind.
And our Lord obeyed His Father out of love unto us, taking upon Him scorn and suffering joyfully, as Scripture says: “Who for the joy that was set before him endured the cross, despising the shame.”
Therefore our Lord said in the night in which He was betrayed: “This is my body which is given for the salvation of the world unto life. And this is my blood which is shed for all for the remission of sins. In behalf of them I offer myself.”
Isaac the Syrian (c. 630-c. 700): Mystic Treatises, 74, in Mystical Treatises of Isaac of Nineveh, trans. A.J. Wensinck).