Paradise Lost is about Adam and Eve's loss of Paradise; their eating of the Forbidden Fruit has often been called the "Fall" (as in, "fall from innocence" or "fall from grace"), so it's no surprise that images of falling occur throughout the poem. The first characters we meet – Satan and his legions – are newly fallen, both morally (they disobeyed God and attempted to overthrow him) and literally (in Book 6 they actually fall out of Heaven and into Hell). Satan's first words to his legions are: "Awake, arise, or be forever fallen." To be fallen, in this poem, is to have sinned, or to have disobeyed God.
It is important to note that in this poem people make themselves fall; there is no "fate" or other force that causes Adam, Eve, Satan, and the rebel angels to fall. Thus, in Book 3 God says that he created Adam "Sufficient to have stood, though free to fall" (3.95-96, our emphasis). And in Book 6, God doesn't push Satan and his legions out of Heaven. They actually throw themselves out of Heaven. Let's repeat that: God doesn't throw them out, they throw themselves out! In Milton's words: "headlong themselves they threw/ Down from the verge of Heav'n" (6.864-865).
In addition to these two falls, Milton also uses a number of other connected images. Now, an object that has fallen is no longer standing; it is no longer upright. In Book 11, Michael tells Adam that "man's woe" (11.632) begins with "man's effeminate slackness" (11.634). Michael implies here that Adam was effeminately "slack" when he listened to Eve and ate the fruit. In other words, he implies that Adam was like a woman, not a man (it is hard not to associate "slackness" with something that is supposed to be hard but is not – wink, wink). Just remember that "slackness" is associated with something going limp, with not being a 'real' man, with the Fall, and with a general lapse in judgment.
If being fallen or "slack" is a sign of one's state of sin, doing the opposite – standing – is a sign of goodness, innocence, and distinction. So, for example, in the passage from Book 3 above Milton's God says that he created mankind "sufficient to have stood," which sounds a lot like "with sufficient capabilities to stand tall and do the right thing." We see the same type of language in Book 7, where Raphael talks about how one distinguishing feature of man is that he is "not prone" (506); in other words, he walks upright, unlike the animals. When Satan first sees Adam and Eve, Milton tells us that he saw "Two of far nobler shape erect and tall,/ Godlike erect, with native honor clad" (4.288-289). In this passage, the fact that Adam and Eve stand "erect" (or upright) is a sign of their distinction as (innocent) human beings; it is what separates them from the animals, and thus makes them closer to God. For another passage, check out 11.509.
In addition, the action of standing up can mean something like what it does nowadays in phrases like "why don't you stand up for something?" So, in Book 9 Adam encourages Eve to keep her reason "erect" (9.353), by which he means something like "stay sharp and do the right thing." In Book 5, when Abdiel realizes that Satan's rebellion is a bad idea, Raphael says he "Stood up, and in a flame of zeal severe/ The current of his [Satan's] fury thus opposed" (5.807-808). At the end of Book 6, Raphael says of the rebel angels: "firm they might have stood,/ Yet fell" (6.911-912). And of course, in Book 3 God explicitly contrasts standing and falling: "Freely they stood who stood, and fell who fell" (3.102). And let's not forget that when he was in Heaven Satan was "upright and pure" (4.837).
Brightness and Light
We never actually see Milton's God; the only real thing Milton says about him is that he's really bright, or that he's like a giant light, hidden away somewhere. One of the ways in which Milton indicates a particular character's virtue is by how "bright" they are. So, for example, Satan refers to the "bright confines" (2.395) of Heaven, and Milton notes that "God is light,/ And never but in unapproachèd light/ Dwelt" (3.305). Other references to God's "glorious brightness" (2.376) are scattered throughout the poem (such as 3.375).
The angels are also always described as bright, luminous beings. For example, in Book 3 Milton says they "Stood thick as stars" (61), stars being some of the brightest objects in the universe (the sun is a star!). In Book 5, Adam sees Raphael approaching from the east (where the sun rises) and even thinks it's a second sunrise ["seems another morn/ Risen on mid-noon," (5.309-311)].
Satan provides a nice contrast. When he lived in Heaven, he was "Clothed with transcendent brightness" (1.86) and "didst out-shine/ Myriads" (1.86-87). Later we learn that, after he fell from Heaven, Satan "his form had yet not lost/ All her original brightness" (1.591-592), a remark that suggests he has lost at least some as a result of his sin. Our suspicions are confirmed by Raphael, who notes that he was "brighter once" (7.132). A similar description occurs at 10.445.
The One Man
The idea of the "one man" is very important in Paradise Lost. It represents Milton's idea of the difference that one man can make – whether literally or symbolically. The poster child for the "one man" is Jesus Christ, the son of God. At the very beginning of the poem, Milton notes that the damage caused by Adam and Eve's disobedience can only be remedied by "one greater man" (Jesus) who will "restore us" and "regain the blissful seat." In Book 3, we actually see Jesus – he's not called that yet because he has yet to become mortal and assumed that name; he's just the Son – volunteer to become mortal and die for man's sins.
Christians believe that Jesus is prefigured (a fancy word that means something like foreshadowed) in Biblical history by a number of Old Testament prophets. So, when Michael gives Adam his little history lesson near the end of the poem, Adam sees "one rising, eminent/ In wise deport" who "spake much of right and wrong" (11.665-666); this is Enoch, a man referred to later as a "just man" (11.681). Later, Adam witnesses everything that happens to Noah, you know that guy who built a huge ark and put every single animal in it while God flooded the world? Well, Michael tells Adam the following: "So all shall turn degenerate, all depraved,/ Justice and temp'rance, truth and faith forgot;/ One man except, the only son of light/ In a dark age" (11.806-809). Finally, there's Abraham, the man whose descendants will be God's chosen people. Michael tells Adam that "A Nation from one faithful man" (i.e., Abraham) will "spring" (12.113, emphasis ours).
Let's forget about Abdiel, the angel who does what all the other rebel angels should have done; he attends Satan's initial council in Heaven before the war (Book 6), but then decides the whole thing is a huge mistake. When he returns to the fold of good angels, Milton writes: "gladly then he mixed/ Among those friendly pow'rs who him received/ With joy and acclamations loud, that one/ That of so many myriads fallen—yet one!—/ Returned not lost" (6.21-25; our emphasis). Among the "many myriads fallen," Abdiel is the only "one" who doesn't make the wrong decision.
In many ways, Adam is a failed "one man." He's the original "one" from whom the human race springs, but he's not as successful as the others. Granted, Jesus is the only really truly successful one, but that's because he's God's son. Adam's sin, however, greatly outweighs the failures of all the other men Milton describes as singular in their excellence.
Paradise Lost is about the loss of…well, Paradise. So it's no surprise that images of paradises abound. First and foremost, we have the Garden of Eden. Milton makes it abundantly clear in Book 4 (our first view of paradise) that this is the best paradise of them all. He mentions a number of famous artistic and literary paradises, only to say that the Garden of Eden is much better than them all. In addition to Adam and Eve's Paradise, there's Heaven. It's really bright there, and it doesn't even really get dark.
Now, you might be saying to yourself, well Adam and Eve lose Paradise too. This is true, but there are two other paradises to compensate for this one. First, in Book 12, Michael tells Adam that if he lives the perfect Christian life (he doesn't call it this because Christianity hadn't been invented yet), if he learns to love God by obeying him, he shall "possess/ A paradise within…happier far" (586-587). In other words, paradise is no longer imagined as a place like Hawaii, but rather as an internal sense of peace or calm that occurs when one obeys God as one should. Finally, at the end of time those who have been saved will be able to live either in Heaven or on earth because the earth will have been turned into a paradise, and a much better one than the Garden of Eden. As Michael tells Adam: "for then the earth/ Shall all be paradise, far happier place/ Than this of Eden, and far happier days" (12.463-465).