It can take years to get some of the jokes. More if you don't read The Bible that often.
War is a woman... oh, sort of like Helen of Troy. Yet another reason for men to kill each other.
"Now"? Remember that as Pepper goes up against War, War smirks and says "little boys with their little toys", and Eve was given the flaming sword—not Adam. War has always been a woman.
Why is War depicted as a beautiful, alluring, desirable woman who, once you get closer, starts to look off-putting and terrifying? Because that's how war has been for the majority of history—glorified, glamourised and romanticised outside of the battlefield, until you're in the thick of it, and then it's already too late.
The only composers Heaven got were Liszt and Edward Elgar, who wrote Land of Hope and Glory, one of England's unoffical national anthems.
The earth is a Libra, because it's the focal point of balance. It houses the forces of Heaven/Hell as well as being the location of their final war, and a main message of the book is humanities capabilities for both good & evil, and how to find balance between the two. Possibly unintentional, but still awesome.
We know Crowley is the serpent from the Garden of Eden—a demon who corrupts humankind by causing them to reject God's instructions and eat from the tree of Knowledge. But Aziraphale, the angel who gives them his flaming sword for warmth, is Prometheus - the Titan who gave the humans heavenly fire, against the orders of Zeus, and was punished for it. Both myths describe a supernatural being in favour of humans having independent intelligence, who were punished by a god jealously guarding it - but Satan is the bad guy, whereas Prometheus is a hero. Two sides of the same coin, as Crowley and Aziraphale are shown to be throughout the book - both rejected from their respective sides of the Balance Between Good and Evil as they both consider that whether humans act for good or bad, they deserve to have the choice.
Anathema's relief upon hearing Crowley address Aziraphale as "angel" was that she was wary of getting raped and assumed it was a term of endearment. It's also driven home when she mentally refers to them as "the consenting bicycle repairmen".
Shortly after Aziraphale turns all the Secret Service men's guns into harmless water pistols at Warlock's birthday party, Crowley does pretty much the same thing but in reverse at Tadfield Manor, which reinforces the mirror image vibe these two give us.
Who, exactly, the Them are. They're the real other Four Horsemen of the Apocalypse, the benign versions of the Rider they took out. Red is War and Pepper is Friendly Scuffles, Chalky is Pollution and Brian is Getting Delightfully Muddy, Sable is Famine and Wensleydale is something along the lines of Forgets to Eat, and while Azrael is Death, Adam is Normal, happy life.
Another example of Azirapale and Crowley being two sides of the same coin: their vague reference to having "resources" actually referred to the same one person.
In another (very punny) instance of "two sides of the same coin" while having dinner together: Aziraphale is eating deviled eggs. Crowley's eating angel food cake.
Aziraphale's name. "Zira" in Hebrew means "brightness of morning." Aziraphale was the guardian of the Eastern Gate of Eden. Also all of the motifs of fire and light worked into his character. The angel Israfel's name is also close enough to be considered a corruption of Aziraphale and means 'the Burning one'. There's also that parking warden's notebook Aziraphale makes combust into flame. And he gifts humans with flame = power/knowledge, essentially making him and Crowley the two daddies of Free Will, and is apparently demoted and assigned to wander the Earth for it, which is not entirely dissimilar to being 'chained to a cliff', if you will, and is likely considered a horrible fate by his fellow angels. The 'fire' and 'light' and Prometheus theme is very real for him.
During Crowley's and Aziraphale's exchange before they face Lucifer: Aziraphale says 'I'd just like to say... I'll have known, deep down inside, that there was a spark of goodness in you.' Look very carefully at the difference in phrasing. In Aziraphale’s phrase, the ‘deep down inside’ comes before ‘that’ and makes no sense if it’s referring to the spark of goodness, as in a spark of goodness deep down inside Crowley. If it were, it should’ve been ‘I’ll have known that, deep down inside, there was a spark of goodness in you.’
The phrasing is correct in Crowley’s response (and it’s appropriate - Aziraphale’s bastard side really is pretty far down inside), but not in Aziraphale’s. Why? Maybe because he isn’t talking about Crowley.
He’s talking about himself. That he has known all along, deep down inside, that Crowley had a spark of goodness in him. And it certainly looks in some parts of the book that he wants to believe in Crowley’s better side despite what he considers his better judgement - but formally he is still very much in the ‘demons evil, Heaven good’ camp for most of the book up till that point.
Aziraphale is essentially apologising for six thousand years of treating Crowley as inherently evil, assuring him that he did know better, deep inside, even if he didn’t always act like it. Setting things straight between them before their expected final battle. Getting the truth out there.
Towards the end of the book, after the Them get rid of the Four Horsepeople of the Apocalypse, Satan is supposed to come. But then Adam- who, keep in mind, is The Antichrist- just waves his hand, and Satan ends up not doing anything. What happens immediately after that? Mr. Young, Adam's adoptive father shows up, and is somewhat cross at Adam for not doing what he said. Adam's father showed up, angry at him for not obeying.
More to support Aziraphale's gayness than his relationship with Crowley, but it's mentioned that he learned how to gavotte at a "discreet gentleman's club." Molly houses (aka proto-gay bars) existed in roughly the same time period as the gavotte (18th century). He also collects Wilde first editions - Oscar Wilde was famously inclined towards gentlemen.
Everything about Adam invokes duality and being neither good nor evil.
An old and largely forgotten trope is to symbolize innocence with gray eyes and blond hair. Gray eyes are often associated with danger and death and thought to be a trait of the Cold Sniper by Civil War soldiers.
Deliberately invoked and thankfully later averted in regards to Baby B.
It would be nice to think that the Satanist Nuns had the surplus baby - Baby B - discreetly adopted. That he grew up to be a normal, happy, laughing child, active and exuberant; and after that, grew further to become a normal, fairly contented adult. ... Possibly you would like to imagine some children, and a hobby - restoring vintage bicycles, perhaps, or breeding tropical fish. You don't want to know what could happen to Baby B. We like your version better, anyway. He probably wins prizes for his tropical fish.