A: You're right ... I did raise this question and then move on quickly without giving any further commentary. Thank you for following up!
The purpose of asking the question was simply to see if that assumption -- namely, that Demons are Fallen Angels -- was held by everyone; which it turns out was the case. The reason I asked the question during our study, was because the Bible never actually states this "fact" clearly. My intention was simply to raise an issue that most of us believe to be true, even though the Bible doesn't teach it explicitly. There are many "truths" of our faith that we hold in our minds that are a product, not of our reading of the Scriptures, but instead of our culture and/or up-bringing.
In this case, being in the church has resulted in everyone around the table of our Bible study believing that Demons and Fallen Angels are the same entities. However, it's possible that this is not true.
In the question above, there are three explicit parts: 1) If demons are not fallen angels, where did they come from?; 2) Have they always been?; and 3) Who created them? All of these questions are excellent. It's possible that I won't be able to answer any of these questions definitively. Sorry!
The reason for this is because Angels and Demons are secondary characters in the biblical testimony. P. Scott dealt with some of the related issues for questions 2) and 3) in his excellent post about the origin of Satan and evil. In that post he wrestles with some of the difficulties of discerning the "origin" of Satan and other details because Satan (and Angels and Demons, too!) are included in the narrative as they relate to the salvation and redemption of human beings by the Triune God. We don't get as much information about these other entities as we'd like, simply because the Book isn't about them!
All that being said, let's try and address your questions in order.
1) If demons are not fallen angels, where did they come from?
While everyone around our table took for granted that demons are simply fallen angels, this is only one theory that has gained support over the course of Christian history. Each of the views have difficulties and there is a good possibility that other views exist with which I am not familiar. As a result of the varied views, it will be impossible to delve into any of these individual views in great detail.
The first possibility has already been raised (demons = fallen angels) and this has been the predominant view since the end of the first century, although it was certainly not unanimous.
A proponent of the second view to be described was the early church "father" Justin Martyr. He wrote in Chapter 5 of his Second Apology that demons were a product of fallen angels having intercourse with human women, with the resulting offspring being "demons." Justin Martyr is alluding to this passage from Genesis:
Now it came about, when men began to multiply on the face of the land, and daughters were born to them, that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose. Then the LORD said, "My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years." The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown. (Genesis 6:1-4, NASB)
This thread of teaching, of which Justin Martyr is only one advocate (e.g. the non-canonical Book of Enoch teaches something very similar, and Athenagoras teaches this explicitly in Chapter 25 of his A Plea for the Christians), believes that after the Flood (Genesis 7), the disembodied spirits of these offspring between the "sons of God" and the "daughters of men" became what the Scripture later refers to as "demons."
A third possibility has been suggested throughout history, and still prevails into our modern day, that there is a "Gap" in between Genesis 1:1 and 1:2 where a "Pre Adamic" (or before Adam) race inhabited Earth. This race was destroyed by God prior to the (re)creation of the world which is described in Genesis 1:2 and following. It's important to point out that not everyone who holds to the "Gap theory" believes that the disembodied spirits of the Pre Adamic race became what we now know as demons, as the "Gap" is also postulated to allow for millions and billions of years into the biblical text for some (certainly not all) who hold to various forms of "Old Earth Creationism."
Watchmen Nee has propagated a form of this "Gap" teaching that believes that demons are the disembodied spirits of this hypothesized Pre Adamic Race of beings that lived during the gap between the first two verses of the record in Genesis. Again, this is speculative at best.
A fourth view is held regarding the origin of demons, at the very least, in common folklore: demons, or "bad spirits," are disembodied spirits of "bad" people who have died or even the result of those dying with "unfinished business." These views are commonly expressed in Hollywood movies and have a profound impact on the views of our culture. This view is also prominent in many developing countries.
Unlike the other views considered so far, this fourth view is more than speculative; it is anti-biblical. Despite the reality that many Christians do believe that the spirits or souls of people can be "trapped" on Earth and haunt people or do other poltergeist-like things, this view is contrary to what the Bible teaches regarding what happens after death. For a more detailed biblical look at why this view is not appropriate for the person who believes the Scriptures, take some time to read these posts about Ghosts and about the present reality of both Heaven and Hell.
A fifth commonly held view about demons is that they originated in the minds of unsophisticated, non-scientific people as a result of ignorance about the way the world works. You may think that it is only atheists who believe that the stories recorded in the Bible are a product of people attributing things like sickness to "malevolent spirits" because they didn't know anything about the real cause: germs. However, this is an incorrect conclusion.
Many modern-day Christians do not think that "demons" have anything to do with our modern world... that's only for people in developing countries and in the "Bible times," but we don't really believe in such things anymore. In this view, everything that was attributed to "demons" in the Scriptures can now be explained scientifically. The idea of "demons" causing any physical ailments in today's world is absolutely laughable to some, particularly those who put a lot of weight on the advances of medical science. In my opinion, the accounts in Scripture make the reality of the demonic world more than a simple explanation of previously unexplainable phenomena. Therefore, I do not agree with the conclusion that demons are no longer applicable to modern, "sophisticated" believers.
There is at least one final possibility: demons are a separate and uniquely created group of spirit-beings. This view is likewise speculative, as there is no passage in Scripture that tells us this is the case. This view is possible in the fact that it does not contradict any explicit teachings of the Bible, either.
Since the Questioner asked for my answer to the question, I would (very hesitantly!) put myself in this final camp, although I could probably be persuaded that the Fallen Angel idea is right.
The main reason I would place myself in the "separate creation" camp is because it seems to me that the Bible gives contrary teaching for how the believer is supposed to interact with demons and with fallen angels.
First, the Scripture teaches that followers of Jesus are given authority over the demons (e.g. Matthew 10:8; Mark 16:17). However, regarding angels believers are given a strict warning not to revile angelic majesties (see 2 Peter 2:10 and Jude 1:8)! To me, there is a disconnect between the power and authority that is given to angels and to demons, which points to some inherent difference of their being. As a separate created group of spiritual beings, this makes sense to me. However, I am the first to admit that this, too, is speculative.
After surveying these possibilities, we can now move on to question 2) Have they always been?
This question is easier to answer: No. Like all created things, demons have a "beginning." It is difficult to determine exactly when these beings were created, especially considering the wide array of views from above. Some believe they were formed in the "Gap", others would claim they were created after the Flood, while still others would claim that they are still currently being created (either via the imagination or in reality as a result of God's judgment or the continued sinful and rebellious choices of human beings)! Still others would maintain that these beings were created during the 6 days of creation along with everything else.
No matter what view we take, the Scriptures clearly teach that God alone is eternal. Every other thing that exists was created either directly or indirectly as a product of God's creative work.
For by Him all things were created, both in the heavens and on earth, visible and invisible, whether thrones or dominions or rulers or authorities-- all things have been created through Him and for Him. He is before all things, and in Him all things hold together. (Colossians 1:16-17, NASB)
Whatever "demons" are, we can be sure that they are accountable to God and that they operate within His sovereign design for the universe. God is not worried about the demons and their plans, as He knows the beginning from the end, and they will all be judged in the proper time. Even the demons acknowledged this before Jesus (see Matthew 8:28-29).
Finally, 3) Who created them?
God's creative act made the spiritual beings that we know as demons. In keeping with the character of God and the revelation of His word, sin corrupts His good creation. Therefore, as with Satan and human beings, sin has distorted these spiritual beings from their previous glory.
In conclusion, this is one of several areas that I wish the Bible gave us more information. However, the Bible doesn't exist to answer all of our questions. Instead, it exists to give us the information we need in order to be saved from this evil generation through the only Savior and Lord of All, Jesus the Christ. As a result, the Bible is content to simply tell us that demons exist and that we should be aware of their reality.
To ignore the demonic realm is foolish. Likewise, to be too interested in the demonic realm is also foolish. We are taught to seek the Holy Spirit and promised that He will guide us in the truth. May we be a people led by the Spirit of the living God, busy with His business, until He returns or calls us home.