Hebrews 13: 2: Be not forgetful to entertain strangers: for thereby some have entertained angels unawares.
When God appeared to Abraham in Genesis 18, He came in human flesh and looked like an ordinary man, but Abraham by revelation realized that the man standing before Him was God Himself. Paul however indicates to us that supernatural beings can appear to us in the form of an ordinary person and we would be none the wiser to it. It is for this reason that we must treat and interact with strangers in a respectful and hospitable fashion, for there is every possibility that an angel from Heaven can materialize in this world in a body of flesh to make investigation of this world, observe our conduct and speak very specifically to us.
When Elijah visited the woman of Zarephath in 1 Kings 17, she showed respect and hospitality to the prophet and obeyed his instructions of taking her remaining meal and oil to make the prophet a small cake. Without her knowing, she was entertaining an angel, not a supernatural being, but the prophet to Israel, just as the seven church age messengers are called angels in Revelation 1: 20: The mystery of the seven stars which thou sawest in my right hand, and the seven golden candlesticks. The seven stars are the angels of the seven churches: and the seven candlesticks which thou sawest are the seven churches. From this we learn that the angels we may entertain can be supernatural beings materialized in flesh, or they may be a man of God sent our way without us even knowing who that person is.