Following from yesterday's discussion; I shall put forward here an explanation for those beings who are traditionally regarded as demons in orthodox Christian theology:
The definition of which is that they are spirit-beings, that have never incarnated, and which are eternally committed to evil such that they cannot be redeemed or saved.
There are several other kinds of being that might be termed "demons"* - and these might include currently-incarnated beings, or beings that have previously been incarnated and died (analogous to evil ghosts)...
But here I am focusing on beings that have always been spirits - and I am trying to understand and explain how any being might be outwith the scope of the salvation of Jesus Christ.
I will first outline my metaphysical assumptions: those who want to cut straight to the explanation of demons will find this after the break: indicated +++
My metaphysical understanding of reality is that it comprises beings in relationships; and that beings have attributes such that they are alive, conscious (to some degree), purposive, self-sustaining and so forth.
Beings are eternal, always have existed - including before creation.
All beings are unique: that is important. Some Christians (perhaps influenced by oneness spiritualty) assume that all Men are originally equal, and diverge - and then spiritually develop and converge towards becoming very similar, or even identical.
But in contrast I assume that all beings began as unique (and un-cooperative) and God's plan of salvation is a matter of progressively achieving harmony. At first a partly-imposed and temporary harmony, but a process that is completed by the Second Creation, of Heaven; where each resurrected being has chosen to be remade such as to live wholly from-love, and harmony between unalike beings is then innate and spontaneous**.
That is: living-beings are the primary unit of reality; and "beings" include humans, animals, plants, as well as what are usually considered to be "mineral" entities and materials.
Everything primary in reality is thus either itself a being, or part of a being.
A further assumption is that all beings were originally spirits.
Only later, and only for some beings, there may be incarnation into some material form.
Many spirits have never been incarnated.
Originally (before creation) beings may have been aware of each other, to varying degrees, and with varying attitudes - but each being is assumed to have been autonomous, separate, pursuing its own goals.
Before creation, reality was a "chaos" of beings.
Divine creation happened when God's love became the means of "coordinating" or harmonizing these already-existing beings.
However, to become a part of divine creation, a being must first be capable of love; then secondly a being-capable-of-love must choose to live by, or from, this love.
(Because it is love that harmonizes the disparate beings, and this harmonization is creation.)
Hence the formulation "God is love", and the great commandments to love God and fellow Men - which can be interpreted to make the choice to embrace the loving purpose of God as ideal, and to aspire to live wholly from love of other beings.
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Now I will explain the origin of demons:
All beings are unique. To participate in creation, a being must be capable of love; but not all beings are capable of love.
A love-incapable being happens to be constituted that way. Such a being will not be part of creation, and cannot ever become a part of creation.
Demons are here regarded as spirits that are incapable of love.
For a being to be saved or redeemed includes death and resurrection such as to live entirely from love.
It follows that any being constitutionally incapable of love, cannot undergo salvation, indeed, such a being cannot even want salvation.
Some love-incapable beings might remain passive, detached from creation. But demons are assumed to be those love-incapable beings who have chosen to oppose creation - for various motives.
For instance; demons may aim to exploit creation, or may seek to destroy it. They may be motivated by their own pleasure, by negative resentment, or sheer spiteful desire to inflict suffering. Or mixture of motivation; and demonic motivations may change over time.
But although capable of changing their behaviours; a love-incapable demon can never become part of creation, and can never seek or attain salvation through following Jesus Christ.
The above scheme may explain the occurrence of those demons - traditionally-defined: demons that are spirits, have never incarnated, and are eternally outwith even the possibility of salvation.
*(And some of these are capable of choosing salvation.)
**(An analogy is that the first creation is like a State ruled by a loving creator God with a mixture of coercion and consent; while the Second Creation is a perfectly loving family.)

