What are angels and demons?
I believe they are invisible spiritual beings to serve one of two masters, the one to damnation, and the other to everlasting life. Every human has spirit, because it is spirit that gives life and the flesh profits nothing (Jo 6:63). The master all of mankind serves, has been determined the day we are physically born. And from the moment man freely chose to heed the voice of the serpent in the garden of Eden, all of mankind is born with spirit (mind, heart, will, emotions) like that of the master we chose to serve, the master over sin and death. All of mankind is born to serve our master Satan. It is for this reason that Christ tells mankind "Ye must be born again." Because to be born again is to be born from above, that is when the old spirit (mind, heart, will, emotions) is made new again through the Spirit of the Lord once again breathing into us a new mind, heart, will and emotions. A new Spirit that will guide us to everlasting life.
When we are born with spirit like the master we serve, it's not that we become spirits! We become spiritual, in our ways of life, our minds, hearts, wills and emotions become like the master we serve. When we are in unbelief, we aren't invisible evil spirits, devils or demons that we serve, but we identify so closely to the way they operate that we are said to be of our father, the devil (Jo 8:44).
The same is true when we become born again. Our old ways are replaced through our new master, who is the Spirit of Christ in us. When we are born again, we don't become invisible spirits, angels, we become spiritual in our ways of life. We so closely identify with the Lord when we are of His Spirit that God has become our Father.
When we speak of humanity as being spirits of devils, demons, or angels of God, we mean that mankind by their actions/inactions prove themselves to be the instruments of power for the master's we serve, by how we live clothed in mortal flesh in this world. When we live in unbelief serving the master over sin and death, our body of flesh is used to deceive, to lie, to steal, and kill etc., there is no limit to what fallen man might do as they willfully serve the master over sin and death. The same is true of all who have been born again. The Holy Spirit in us uses our mortal body to demonstrate to the world the fruit of the Spirit of Christ in us, that unbelievers might come to know the joy and peace that await all who have been born again.
Galatians 5:19-25 (KJV) Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God. But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, Meekness, temperance: against such there is no law. And they that are Christ's have crucified the flesh with the affections and lusts. If we live in the Spirit, let us also walk in the Spirit.