In the Beginning
There are two things all angels know about God.
The first thing is that She does not play dice with the universe. The Almighty plays an ineffable game of her own devising. For everyone else though, it's like playing poker in a pitch dark room for infinite stakes with a dealer who refuses to tell you the rules and smiles all the time.
The other thing is that God always has a plan. But, it works in mysterious ways and no one can really understand it. Most angels are fine with only knowing that it will work in the end, especially since they all know what happened the last time someone even dared to question Her.
Except a certain Principality, that is.
Aziraphale always like to collect these little facts and know how things work or what they are. He doesn't question, per se. He just likes to dig deeper and find the context behind things.
So when She handed Aziraphale a flaming sword and ushered him to the wall, telling him to guard Eden and the tree placed in the middle of it, he stood there baffled as She proceeded to leave him with no further instructions.
"Why must I guard a tree?" he muttered before turning to look at the entirety of Eden. Something rose in him; not doubt, but curiosity.
It made him restless. For the past two days on Earth, he can't stop thinking about it. Aziraphale comes up with various answers in his head as he tended to the Garden, or simply went around exploring and admiring Her creations.
the first two of them actually.
On the fourth day, Aziraphale was sitting below the exact same tree that he has been thinking about for the past days. He lazily twirled his flaming sword, watching as the flames crackled and glowed.
Aziraphale looked up to the tree's leaves, marvelling at the way the green leaves seemingly glisten in the sunlight. Everything She created really is beautiful. Then, his gaze landed on a red, juicy looking fruit. His mouth started to water. He can't recall tasting this one yet. What was it again? Maybe a nibble could remind him...
Aziraphale was about to reach out and pluck one off the branches when a voice spoke up.
"Principality?" Eve said, approaching him slowly. Aziraphale retracted his hand before standing up.
"Eve!" He greeted, giving her a kind smile. The angel noticed that her usual companion, Adam, was nowhere to be found. He looked around. "Where's Adam?"
"Naming the last few animals. He's still torn over what to call the ones with the long necks." She replied, approaching him with a bit of hesitance. Aziraphale could sense that she wants to say something but keeps her mouth shut.
"What's on you mind, Eve?"
"Can I ask you a question?"
Aziraphale nodded, "I will do my best to answer."
"Why is this one forbidden, exactly?" She asked, pointing towards something. When Aziraphale followed her finger, he realized that she was referring to the tree.
"Yes. God said that we could eat every fruit in here except the one that this tree bears." She said. She turned to face Aziraphale. "Do you know why?"
Something in his mind clicked. He mentally facepalmed, not knowing how could he have missed it. The answer was in front of him all along! He was sent to protect this tree because it was the Tree of Knowledge!
 Aziraphale has heard about it around heaven during Earth's creation, but never knew what it looked like. But what he did know is that it contained the knowledge between Good and Evil and that Almighty was going to put it somewhere in Eden.
"Well," Aziraphale cleared his throat, relieved that Eve interrupted him from picking the fruit. Who knows what could've happened if he did that. "It may be forbidden because it contains...certain knowledge that only the Almighty knows." He said, trying to avoid saying the words bad or evil .
"Really?" Eve said, eyeing the fruit. "Is that why it is forbidden? Because God does not wish to share such knowledge to Her creations?" She pondered in a solemn tone. Aziraphale did not really know how to answer that.
"Thou shan't worry nor doubt, for everything She does has a reason." He said, repeating the words he always heard from the other angels when he was the one to ask questions.
Eve didn't look satisfied with that answer, though. Her hand was itching to reach up, to take a bite and to know just what kind of knowledge was kept in that fruit-knowledge so intriguing that the Almighty kept it only to herself. However, she stayed still, for the time being.
Aziraphale nodded once. "Do stay away from it, Eve. No matter how tempting it may be to reach up and bite from that...that..." He advised, faltering as he tried to remember what the name of the fruit is.
Eve tilted her head. "It does look like an apple."
Aziraphale hummed. "Then stay away from that Apple, then. Have a wonderful day. Oh, and do send my regards to your companion." With one last smile, he spread his wings and flew away, back to the wall where he was actually placed.
Eve watched the celestial being fly off, still thinking about what they have talked about. She silently wondered what was so interesting that it was declared off limits to everyone except Her.
Eve walked away, and did the same for the next two day.
Until the seventh day, when she couldn't resist the fruit anymore.