Anthony,

Sorry for the long delay in replying; sometimes things fall between the cracks.

St. Isaac of Ninevah, also known as St. Isaac the Syrian, is a greatly respected desert father who belonged to the Assyrian Church of the East. As such, he was never in communion with the Catholic Church or the Orthodox Church, but people in both churches venerate him greatly, including myself.

I do not know if he was a heretic as we would define it; I am not aware of any of his teachings that are problematic or anything he engaged in that would be heretical.

Perhaps St. Isaac was exaggerating to make a point, that our hearts should be so loving and merciful, that they should know no bounds. From a Catholic perspective, praying for demons, in the sense of praying for them to receive mercy and protection, is problematic, as they cannot repent and will never be reconciled to God since they reject God's mercy. They have made up their mind and their will is fixed. According to ChatGPT, the Assyrian Church of the East's views align with this. Any love for them would likely be perceived as pain on their part.

I will note that he doesn't technically say here that we should love and pray for demons. He says we should have a "heart burning for the sake of all creation . . . for demons . . ."

Perhaps by this, he simply means that we should wish for their redemption despite its impossibility; that we should regret their state and mourn their loss.