This is theory I've had for a while, I touched on it a bit when I did the anime contract history, but this goes into a bit more detail.
* Note: If you haven't read the manga I make a few references to its (slightly different) version of events. All you really need to know is this: The murder of Ciel's family and the Ritual that resulted in Sebastian being summoned are unconnected events. The mystery murderers, instead of killing Ciel, got greedy and kidnapped him and sold him to human traffickers. They then sold him to the cult. The cult branded his back with a
snake-like symbol, held him prisoner for about a month, and then used him (and multiple other children) in what appeared to be a botched demon summoning.
* I noticed something kinda weird about Ash/Angela. Angela's motives are seemingly consistent and so are Ash's...but Angela and Ash's motives aren't consistent with each other. Angela has an interest in Ciel that I go into in a lot of detail here and, in general, she behaves in a proper angelic way. Ash, on the other hand, wants to eliminate all filth (including Ciel). He's also a total nut-job and probably fallen. I kinda wonder if, at one point they were separate, either two Angels (one pure, one fallen) that have a bit of an alliance or just a split-personality thing. Either way, Until I see conclusive proof that they were always one in the same, I'm refering to them as separate people. Because it actually makes sense that way.
Anyway, here we go.
The Phantomhive manor catches on fire and Ciel finds his parents dead. This occurs on Ciel's 10th birthday.
They were murdered by Angela.
Somehow, Ciel ends up in the hands of a cult (despite a line in episode 14 giving lip-service to the one month of captivity thing in the manga, all other dialogue points to this occurring on Ciel's birthday, the same day as the fire/murder). They are wearing white robes (not
black). And there is a conspicuous lack of other children.
They place Ciel on an alter that's in the center of an odd symbol (not a
pentacle).
Ciel is branded with the same symbol.
He is then "sacrificed". Presumably, he also
rejects God at this moment.
Sudenly he's floating in a mysterious place, surrounded by white feathers.
A demon appears and says he cannot enter heaven because he has rejected God and offers a contract.
Ciel accepts and the feathers turn black.
He is marked by the demon and orders the death of the cultists.
Fast-forward (exactly) 3 years and Ciel find that same odd symbol in a mansion being used by Drocell's master.
Said master (who has Ash's voice), has specific knowledge of what has happened to Ciel. Angela is connected to this master in some way.
Sometime later, while investigating a strange convent, Ciel sees that they use that same odd symbol.
It is revealed that Angela is in control of the cult, meaning that that symbol could represent her followers. The symbol that was used in the failed ceremony Ciel was involved in, the symbol he was forcibly branded with. Which answers the question of how Ciel got from the burning mansion to the cult, Angel took him to them. This implies that the "ritual" was set up by her. And the fact that she/Ash is responsible for convincing the queen to kill his parents and the fact there are no other children present suggests that Ciel was specifically selected for this ritual. Which begs the question of just what the ritual was suppose to be.
Angela promptly kidnaps Ciel, who's Cinematic Record she appears to have stolen. Her goal is to "purify" Ciel. It fails. Presumably, had she succeeded, she could have broken his contract with Sebastian and probably "restored" his faith in God (I have the feeling that his lack of faith is what made Angela's original ritual fail).
She appears again, when Sebastian is arrested, this time trying to get him to break the contract between him and Ciel. And, here's the important part, she even goes so far as to offer him other people's souls, as many as he wants, if he just "removes his hands" from Ciel. If her goal was just to save a soul from a demon she wouldn't try to do it by giving him other souls. She clearly specifically wants Ciel's soul and doesn't care what happens to others. That goes back to the point above, there is something special about Ciel in particular (or at least his soul). Which also goes back to the whole deal with Sebastian only wanting high-quality souls (he even says Ciel's is the greatest).
In conclusion, in the Kuro (anime) universe, there is something special about Ciel Phantomhive's soul and at least one angel is interested in using that soul for some purpose.