In the beginning, there was a super intelligent being, called God. Except there was no beginning, because God had always existed. Forever.
One day, God decided to create a universe, as you do. He made it full of about 2 trillion galaxies, each containing about 100 billion stars, and around many of the stars he created several planets.
Then he picked just one planet, and created all living things on it.
I have to stop here and back up a bit, because there are two versions of the story, each with different implications. The vast majority of Christadelphians would claim there is only one story, but I'll be generous and pretend that the second story, with its few hundred adherents, is not merely statistical noise in the overall Christadelphian Story.
Creation - Version 1
There are also two versions of the first story, so I'll call them 1a, and 1b.
In version 1a, God created all living things in 6 days, or perhaps 6 lots of some unspecified length of time, depending on who you talk to. There isn't much else to say here, because the details are somewhat fuzzy.
In version 1b, God never actually created the aforementioned galaxies, stars, and planets, because if he had, the light wouldn't have had time to reach us, meaning we wouldn't be able to see them.
But we can see them...
So what God must have done instead is create the photons of light already on their way to Earth, so that it looked like he had created galaxies, stars, and planets, but without there actually being galaxies, stars, and planets. Or maybe he really did create them as well, and then sped up the light photons when he realised we'd never see them. Either way, many of the stars had already exploded long before the light reached us, so by the time we saw them they were no longer there anyway. Oh well.
He did all of that in less than 1 day, but then took 6 days to make all the plants, animals, viruses and bacteria on earth. Oh, and he created the sun on day 4. What's wrong? Why are you looking at me like that?
The story continues below - see the heading 'Round 1'.
Creation - Version 2
Rather than creating all of the galaxies, stars and planets in one go, God rolled them out over about 13.8 billion years or so, and is still going with it, making new stars regularly. Or, more probably he just created the mechanisms that allowed them to form on their own. Clever.
Then about 4.5 billion years ago he decided to create life on one planet, and for about 3 billion years he just played around with single celled organisms. Then one day he decided to make organisms with more than one cell, and before long (relatively speaking) he was making animals and plants. Unfortunately more than 99% of them are now extinct, but the important part is that despite at least 5 major extinction events, he eventually got around to making humans, which was apparently the whole point of making a universe in the first place. Lucky us. Then he let them roam around for about 200,000 years, in pretty desperate circumstances, and with a pretty high mortality rate.
Whether he guided the whole process of life's development or just mixed the chemicals to start it off, we're not sure. Likewise, we don't know whether he made humans specially, or just some of them (presumably he specially made Adam and Eve to look exactly like the ones that had already evolved, complete with all of the same genetic defects. We're not told why).
At some point God decided to interact with humans, probably within the last 10,000 years.
This is where the two versions of the story now continue on in more general agreement.
God made 2 people, a man and a woman, and put them in a garden. He gave them 1 rule that they had to follow. One of his other creations, a snake, literally talked them into breaking that rule, so he banished them from the garden and decided it was a better idea if snakes couldn't talk. He also removed the legs of all snakes (why?), although that didn't seem to affect their success as a predator that much.
God then cursed the ground (why?) and made everyone mortal, including all animals (again, why?).
Adam and Eve had 2 children, but one of them (Cain) killed the other one (Abel) so God sent him away. At first Cain tried suggesting that he needed protection in case someone killed him (who?). Then he went away, got married (to whom?), built a city, and generally did pretty well for himself.
So Adam and Eve had another son, who had more kids (how?), and before long (relatively speaking) the world's population started to grow. But for some reason most of them turned out evil.
So God drowned them all (or just some of them, depending on who you talk to). Except for this one guy (Noah) and his family. He had Noah build a boat, so he could survive the coming flood. Apparently no one else had boats.
And so several million species (plus viruses and bacteria, and all known terrestrial plants, but not dinosaurs) boarded a boat for a year, and didn't eat each other. Or perhaps it was several thousand species who then speciated extremely rapidly in the ~4000 years since. It's hard to tell.
God then promised not to flood the earth again, and showed a rainbow as confirmation. This was before people understood how light refraction worked.
So Noah's sons had children, who then also had children (who with?), and so on. Stop asking questions. Moving right along...
Then people started building a tower in order to keep society unified, but God didn't want that so he changed their languages and scattered them across the whole world. I'm sure he had a good reason.
But there was this one guy, Abram, who God took a liking to, and promised him as many descendants as there are stars. Given the actual number of stars in the universe (see above), I'm guessing this was hyperbole. As the story goes, (some of) this man's descendants became the nation of Israel, and so from that time on, God played favourites and did generally nice things for that nation, while either ignoring or punishing all the others. Why they were punished, we are not told. Either God made himself known to all the other nations as well, and gave them all commandments, or he punished them for doing things they didn't know they were not supposed to do. Sometimes, actually a lot of the time, his favourite people also did things they were not supposed to do, so he punished them too.
Eventually God's favourite people played up one time too many so God sent some of them off to Assyria and the rest to Babylon. The ones that went to Babylon later returned to Israel, but we don't know what happened to the others.
One day, God decided to have a son of his own, who would be human. Bear with me.
After what happened with Adam and Eve (see above), God wasn't happy with people any more. Apparently when the people he made did things he didn't like, he thought they should die. In my line of work, when the software doesn't behave the way the programmer intended, we call that a bug. In any case, it seems that when humans behaved the way humans naturally do, God wasn't happy about it, and the only way he could be appeased is through bloodshed. I don't know why.
So therefore Jesus, God's son, had to live a perfect life, and then die, in order to reconcile people back to God. And what better way to reconcile people than to have Jesus brutally tortured to death for (perceived) insurrection! But that wasn't enough. God also required people to perform this cleansing ritual where they pretended to die, which was a bit like Jesus actually dying (but without the whole crucifixion thing), and so they were not guilty any more, and God could be happy again.
Up until now God had commanded people to kill animals instead, which was supposed to point forward to the tortured death of Jesus, except that probably no one understood that until after Jesus. As long as something got killed, God could be happy again, or so it seems. But after the death of Jesus, God didn't need people to actually die any more. Apparently if they just pretended to drown for a few seconds, he was cool with it. They died symbolically, but Jesus died for real, so somehow that meant God could forgive people.
It's a bit of a mystery how or why this is supposed to work. Some versions sound like God found a loophole in his own rules and exploited it. Personally I'm a bit suspicious of the fact that this started out with animal sacrifices to appease an angry deity, which was a pretty common theme among almost all ancient religions. Apparently that's just a coincidence though.
So anyway, after Jesus died he went off to heaven, wherever that is.
The second book
A few decades later, God realised that hardly anyone knew who Jesus was, and so he decided to get some people to write it all down. Up until now he had gotten people to write stuff down in Hebrew, but just to mix it up a bit he decided to do the second part in Greek. Of course that meant that people needed to be able to read two languages in order to read the whole thing, but luckily some people had already translated the first part into Greek. So now people only needed to be able to read Greek, in a time when less than 10% of the (local) population could even read at all.
And there was only one copy. But the printing press wasn't going to be invented for another 1400 or so years, so God made sure people made lots of copies of his book by hand. Unfortunately they made lots of mistakes, so we're no longer 100% sure what the originals actually said, but God thought it would be totally fine.
According to Christadelphians, it wasn't fine. Christianity became corrupted (and thus heretical) almost immediately after the book was written, and the true message wouldn't be rediscovered for another 1750 years or so! Of course, there must have always been a few smart readers here and there who managed to get it all correct, because otherwise it means no one would have been saved during that whole period, and that would be ridiculous LOL.
Then God realised that lots of people didn't understand Greek, so he had the book translated into other languages, several times. This led to issues where different translations said different, and sometimes contradictory, things (still totally fine, though).
Thus, Christianity became even more fragmented, and all hope seemed to be lost until the 19th century, when a brave hero survived a near-shipwreck, then later discovered the one true message of God's book that virtually no one else had been able to properly understand for more than 1500 years. Hooray!
The one true faith
Ever since the reformation, every man and his cat was also claiming to have rediscovered the truth, but our hero, John Thomas, is obviously the only one who really did, and all the others were frauds. We'll conveniently ignore the fact that most other protestant denominations say the same thing about their own founder, and believe their own teachings with equally strong conviction.
Christadelphians acknowledge the existence of other religions, but they know that only Christadelphians have the truth and all other religions are false. We will again conveniently ignore the fact that all other religions make similar claims, and with equally strong conviction.
Christadelphians can prove that they are the only ones that have the truth, and they can also prove that all other religions are merely the inventions of men. We will again conveniently ignore the fact that all other religions make similar claims, and with equally strong conviction.
Most Christadelphians are pretty sure that the religion their parents taught them just happens to be the world's only true religion. We will again conveniently ignore the fact that all other religions make similar claims, and with equally strong conviction.
Christadelphians think their religion is the only one that makes sense, and that all of the others are mistaken or deceived. We will again conveniently ignore the fact that all other religions make similar claims, and with equally strong conviction.
God's struggle with humanity
So hopefully you can see now that right from the start God has had an awful lot of trouble convincing the people he made to do what he wanted, despite even offering them eternal life in paradise. No matter how much pain and suffering he inflicted on them, they still wouldn't listen. He drowned some, burned others, and slaughtered even more. He even had his own son killed for them. What will it take to convince people he loves them and wants to save them?
God also had a lot of trouble even getting his message out to people. For most of human history, the vast majority of people had no idea God even existed, let alone that he had a message for them. Most of them believed in and worshipped the wrong god, and couldn't tell the difference between a divinely-inspired book and a human-written one. Or rather they thought the human-written one was better. In fact, at least two thirds of the world's population still can't tell which religious book is the one God wrote.
For Christadelphians, it's even more puzzling. Even among the people who correctly identified God's book, 99.9% of them still cannot understand it properly. They even have the nerve to claim that Christadelphians don't understand the atonement!
But Christadelphians are not about to give up hope. Their religion is built on hope (evidence, not so much). They will continue to preach God's good news to the world, in the hope that a few more might join the one true faith. It's not their fault that most people just don't want to listen. It is of course a godless and wicked generation (thus shifting the blame to those who don't believe, rather than accepting responsibility for failure to convince them).
We will again conveniently ignore the fact that all other religions say and do similar things, and with equally strong conviction.