Hebrews 13:2 Be not forgetful to entertain strangers: for thereby some have entertained angels unawares.
Creation of angels
God created them before He created the world (Job 38:4-7). The fallen angels sinned sometime after creation and probably before Adam and Eve (Eze 28:13-15); many rebelled against God but we do not know exactly how many (Rev 12:3-4). Fallen angels we call demons while satan is called “devil” (Matt 4:1 – diabolos; Matt 9:32 – daimonizomai
Satan is the leader of the fallen angels (Matt12:24, 26) and he apparently has a throne (Isa 14:13). The devil works to deceive us thru various ways – lies (Jn 8:44); possession (Matt 12:22); false doctrine that appeals to people as real light (2 Cor 11:14); physical limitations (Lu 13:16); entrapment (1 Tim 3:7; 2 Tim 2:26); fear & force (1 Pet 5:8).
The devil will use God’s deliverance to entrap people into false doctrine and worship (Col 2:18) causing people to think wrongly about God’s messengers. People do not become angels when they die (Matt 22:30). Angels are always referred to in the male gender, never is the feminine.
What are angels like?
Angels are spirit beings (Heb 1:7, 13). They are very powerful and when seen in their angelic form very intimidating (Dan 8:16-17; Matt 28:2-3). They can take on the appearance of men just as they did in their visit to Sodom and to Lot. God may allow them to appear to us in a form that appeals to us in order to lead and guide us.
Even Jesus appeared in another form after the resurrection (Lu 24:15), but their eyes were beholden – retained; held back. Mark tells us He appeared in another “form” – rearrangement of parts. Angels that appeared at the tomb appeared as young men (Mk 16:5; Lu 24:4). At Christ’s ascension angels appeared as people (Acts 1:10).
Angels’ function
Angels perform many functions for God and the devil. God’s angels basically are messengers or ministers. As messengers, they deliver messages to mankind (Dan 9:23 ff; Jud 6:12; Acts 27:23-24; Lu 1:11, 28). As ministers they perform differing types of service ranging from destroying the enemy (2 Ki 19:35) to shutting the mouth of lions (Dan 6:21-22); delivering out of jail (Acts 12:7-10). The angels of God (and demons) are invisible unless we are enabled to see them or they make their present visible (2 Ki 6:17).
The demons perform similar tasks as the angels of God only for the devil. They hinder (1 Thes 2:18), obstruct the gospel (Acts 13:6-8), they inspire and direct false teachers and doctrine (2 Cor 11:14-15; 1 Tim 4:1).