Most or all of the community here have made the claim that Lucifer is another name for the devil. That is debatable from a high-theology, strictly originalist textual analysis; in the Christian holy texts, the word name Lucifer is only used once in the Bible, where an ancient Hebrew prophet was labeling the king of an enemy nation, calling him the Morning Star. This was more or less sarcastic, obviously. (See Isaiah 14:12) That particular biblical passage does not state, or even imply, that it is referring to a demiurge or evil mythological being.
But that name eventually became applied to the devil in Christianity (and possibly other religions), and the name stuck. That is in part because other passages associate the devil with celestial/meterological phenomena (Jesus: "I saw Satan fall as lightning from heaven" in Luke 10:18; Paul: "the prince of the power of the air, the spirit that is now at work in the sons of disobedience" Ephesians 2:2). John Milton, in his classic Paradise Lost, associated Lucifer with the Serpent in Eden, who enticed the first humans into acquiring first-hand knowledge of good and evil. In this interpretation, the Serpent was, indeed, a Promethean "bearer of light", bringing forbidden knowledge that G-d would have preferred to have been kept concealed.
In any case, it is clear that Boehner intended to call Cruz the devil. What is more significant is that he used the modifier "in the flesh". It is not a sexual phrase; it does not mean "in the nude" or "inside a vagina" or anything similar, although it could certainly appear as such to a non-native English speaker, from a non-Christian background. Indeed, the phrase "in the flesh" has a much more serious connotation.
You see, in the first chapter of the Gospel of John, we are told of "the Word", which simultaneously was "with G-d", and "was G-d" (mind-blowing Trinitarian esoterism). The Word made, or caused, everything that exists; the Word is YHWH. And, the Word (meaning, YWHW himself/itself), who up until that point, had been an invisible, universal cosmic force, decided to become a physical being, with a size, weight, and color. It was at that exact moment in history that Jesus Christ of Nazareth was conceived in the womb of a literal virgin woman two thousand years ago. According to the Gospel, Jesus Christ was "the Word [which] was made flesh" (John 1:14).
A similar concept exists in Mahayana Buddhism; the last of the Four Encompassing Vows made by a Bodhisattva is, "Virtue is inesteemable; I vow to embody it." You see, a Bodhisattva does not promise to be virtuous, or to perform virtuous actions; he promises to become (to become reincarnated as) Virtue itself. He "attains the body of a Buddha" rather than the body of an animal or human being. The Bodhisattva is Virtue, made flesh.
To call somebody "Lucifer" is to call somebody the devil; but that is nothing. That could be simply an exaggeration or metaphor. To call somebody "Lucifer in the flesh" raises the stakes significantly; it suggests that the person is the devil, wholly and completely, having no other qualities apart from it; just as Jesus Christ was literally the physical embodiment of YHWH, the maker of the cosmos; just as the reincarnated Bodhisattva is literally the physical appearance of unconditional virtue itself; just as the consecrated Eucharist is literally transformed into the state-executed remains of YHWH's human avatar.
