Our Torah portion opens with Abraham and Sarah being visited by three angels. At their first appearance, it says that “[Abraham] lifted his eyes and saw three people…” Initially Abraham and Sarah assumed they were human. Only later do Abraham and Sarah find out that they were angels.
Upon leaving Abraham and Sarah’s tent, two of those three angels continued to Abraham’s nephew, Lot’s house, in Sodom, to rescue Lot and his family before the pending destruction of that city. Unlike with their first appearance to Abraham, when the visitors appeared to Lot, they were immediately called angels, not people.
One might initially think that this shows greatness in Lot: Abraham initially thought they were human beings, while Lot could immediately tell they were angels. In truth, though, it is understood to indicate the opposite. Due to Abraham and Sarah’s holy lifestyle, they saw angelic existence as attainable by humans; the visitors didn’t appear to be that different from themselves, so Abraham misidentified them as humans. Lot, though, did not live a holy lifestyle. The difference between the visitors and him (and those around him) was enormous, so there was no confusing their being angels.
We are encouraged to strive to emulate Abraham and Sarah. The more we live in holiness, the more everything in the spiritual realm should seem familiar to us. We bring Heaven down to Earth.