Angels. They exist. They were created by God and they serve God. There are multitudes and multitudes of them. (Heb 12:22).
The bible shows us that angels appear on earth as men, sometimes looking like ordinary men (Joshua 5:13-14; Mark 16:5), and at other times looking like something other-worldly, so much so that the people gazing upon them were gripped with fear, as was Zacharias in Luke 1:12. He was speechless before the angel who visited him. So were the keepers of Jesus’ tomb, 'who became as dead men when they saw the angel of the Lord' (Matthew 28:4).
When they appear to people on the earth sometimes they take on human form like in Genesis 18:1-19. Jesus and two angels appeared as men and actually ate a meal with Abraham. Later two of the angels went on to Sodom and slept overnight at Lot's house. So we know they can incarnate, appear as men, and spend a period of time on earth.
We know that some angels fell, meaning, they sided with Satan in the angelic rebellion and were cast from their places in heaven. Satan's fall is described in Isaiah: "How you have fallen from heaven, O morning star, son of the dawn! You have been cast down to the earth, you who once laid low the nations!" (Isaiah 14:12)
A third of the angels fell with satan: "Then another sign appeared in heaven: an enormous red dragon with seven heads and ten horns and seven crowns on his heads. His tail swept a third of the stars out of the sky and flung them to the earth." (Rev 12:3-4).
Though satan was cast out, he is god of this world (2 Cor 4:4) so being cast out doesn't stop satan from his intent to deceive us. He still pretends to be on God's side. "And no wonder, for Satan himself masquerades as an angel of light." (2 Corinthians 11:14). Note the word 'masquerades' and 'light'. Masquerade means "An involved scheme; a charade; as in wearing a mask or disguise'. Of course he would pretend he is one of the good guys to an unsuspecting unbeliever, or even to a Christian. "It is not surprising, then, if his servants masquerade as servants of righteousness." (2 Cor 11:15). His servants are the fellow angels that followed satan in the war against God and also pretend to be one of the good angels. But they only want to deceive.
Hebrews 13:2 says we sometimes entertain angels unawares.
So the punchline is, if you were beset by an angel of light, how would you know he is one of the good guys, or one of the bad guys? There is a 1 in 3 chance he is one of the bad ones. How would you know? Would you be so gripped with fear that you just accept what the angel says? Would you be so filled with pride that one of these beings was sent to you that you would accept what he said without question? Would you be so entranced by their light that you unquestioningly accept they are from God?
The only way to know is to know the Word of God. John and Peter both warned of false teachers, and Jesus said they may come to us in sheep's clothing (Matt. 7:15; 1 John. 4:1; 2 Pet. 2:1). John said our duty is to "test the spirits," and Paul said: "Test everything. Hold on to the good." (1 Thess. 5:21).
Not all angels come in peace. Some are your enemy. Be watchful and test all things!