First of all, to avoid confusion on this discussion:
"Ciel Phantomhive"=Ciel Ciel Phantomhive=Earl/the Earl [Since he's the current Earl] Sebastian Michaelis=Sebastian
This is just a PURE THEORY of mine so, let's get started.
Having the "Ciel's Twin Theory" confirmed by Yana Toboso herself, there were many questions that got formed in my mind, like, "What's Earl's real name?" and "How come Ciel is Alive?" Because of these questions I came up with a theory. Reading Yabucca's hypothesis about Sebastian, "That Butler, Messed Up?", made me think that "Oh, maybe Sebastian didn't eat Ciel's soul at all", maybe you asked "Why? He became a sacrifice right?" Here are my reasons:
1. Sebastian is picky when it comes to souls
Demons are hated by Grim Reapers because they are eating the souls that the Grim Reapers are about to collect. On the Circus Arc, Chapter 26, Will called Sebastian a "Starving Demon" and he told Will that he is not interested in poor-quality souls. From there, we can say that, maybe Ciel's soul is a little poor for Sebastian's taste which leads to his soul not being eaten and he just got killed for nothing.
2. The occult that killed Ciel as a sacrifice
On the Blue Cult Arc, Chapter 136, we can say that the occult tried to sacrifice other souls too, but actually failed to call the Lord of Chaos, Sebastian. How can I say that? All of them got shocked when an earthquake happened, if they succeeded on calling him before then they shouldn't be shocked and scared by how he will come and what he will be able to do. We can now say that, "CIEL IS NOT A SACRIFICE". Like the other kids that they killed, Ciel just became one of them, if he got sacrificed then his soul shouldn't be there anymore! Sebastian should've eaten his soul but he didn't and I think it is not because of Ciel's soul that Sebastian came.
3. He came because of Earl's words
On the same arc and chapter as the second reason, the earthquake came after Earl said that no one is going to save them and there is no God and it continued until he said that he will kill them all. Sebastian showed up after he said those words, but, where did he came from? He came from the blood of Ciel that dripped from the table, meaning Sebastian only used Ciel as a tool to come out and talk to the Earl. He even looked around and asked who summoned him, meaning the one who summoned him is not a part of the occult, until he saw Earl and was convinced that the Earl was the one who called and summoned him.
4. Sebastian lied as soon as he talked to the Earl
How can I say this? Yabucca said that maybe Ciel is still alive when Earl and Sebastian formed the contract. I doubted that when I first saw it but then I realized, "He may be right". As far as my reasons state, Sebastian is picky when it comes to souls and "maybe" he didn't ate Ciel's soul. But he said that Earl sarcificed something big, well, Will said on the same arc and chapter as the first reason, that demons use flattering words to lead their prey to them, and as soon as Sebastian saw Earl, he used that technique so he can lead the Earl into the hole that he made. He convinced Earl that he is now alone and Sebastian is the only way he can escape even though Ciel is not really dead.
5. Sebastian wanted a soul of revenge
Sebastian is never interested on poor souls, nor he is interested on the sacrifices that the occultists offer, he isn't even interested on the Earl. But as stated earlier, it is not because of the sacrifice, but because of the words Earl said that Sebastian showed himself. Meaning his interest just got to the Earl because he wanted revenge, because he wanted to kill them all and he didn't believed in God. Sebastian wants his soul that became full of revenge, that's why he formed the contract with Earl. He doesn't care about other things, he's just interested on the soul of Earl, that's why HE LIED.
6. Grim Reapers
Well, think about it, Grim Reapers are there when there is someone about to die, right? Not that far isn't it? There's a Grim Reaper there for sure, but is not shown. Who do you think it is? For me, Undertaker may be the one who's in there, he's the one who collected the soul of Claudia Phantomhive after all, or maybe someone unknown. Grim Reapers decide if a person is going to die or live. Maybe the Grim Reaper decided to let Ciel live after watching his records.
That's all for now. Please don't get confused even if I can't explain myself well. Here is a summary so you won't get confused.
1. Sebastian doesn't like poor souls.
2. The occult killed Ciel for nothing.
3. Sebastian showed because of Earl's words.
4. Sebastian lied to Earl that he lost everything.
5. Sebastian likes a soul of revenge.
And lastly,
6. A Grim Reaper may be the reason why Ciel is alive.
Please leave a comment so that I can also know (?/ there's a more appropriate word but I can't think about it) your opinions. Thank you!