Symbolism, Imagery, Allegory
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.