The Butler, Paying a Call is Chapter 136 of the Kuroshitsuji manga.
"Ciel" and Ciel Phantomhive are still with the cult and are continuously abused and violated. Eventually, the "mark of the noble beast" is burned into their backs. Afterward, Ciel wakes up screaming and wonders if the branding was just a dream or not. However, he quickly realizes that it wasn't when he feels the mark on his back. "Ciel" asks him if it still hurts and takes a look, stating that the swelling seems to get better. When Ciel asks "Ciel" if his mark still hurts as well, a drunken man arrives and kicks against the bars of their cage, telling them to be quiet as it is in the middle of the night. Complaining that he has to look after the kidnapped children on Christmas Eve, the man goes away.
"Ciel" cannot believe that they have only spent ten days with the cult, and Ciel asks his twin if he remembers what happened two years ago on Christmas: "Ciel" told Ciel, who believed in him, that Father Christmas did not exist, upsetting his brother. Wanting to find out if his brother was right or wrong, Ciel later hid and waited for Father Christmas to come. However, he only caught Tanaka setting out their presents. After reminding his twin of that Christmas, Ciel proceeds to say that "Ciel" is always right, and that, if there's no Father Christmas, there surely is no God either.
The twins spend one month with the cultists; one day, when the wardens come to force feed the caged children, they notice that one of the children died and take him away. Seeing this, Ciel wonders if "on one of these days, [he'll] join him," but "Ciel" assures him that all will be fine as he will always be there to protect him.
On the next new moon, the cultists hold another of their "masses." The twins are kept in a cage and ponder what is going on as it seems that the cultists did not intend to violate them again. Then, one cultist comes to their cage, opens it, and takes "Ciel." Ciel tries to reach out to his brother and calls after him, but the cage door is quickly closed again, and "Ciel" is dragged onto an "altar."
When "Ciel" is placed on the "altar," the cultists pray to the devil. During their prayer, Ciel continues to call after his brother and after help, but his words accomplish nothing and he is left to watch his beloved twin brother be stabbed in the chest and sacrificed.
Ciel now fully embraces that there is no God, and the building shakes when a demon is summoned. The demon attacks the cultists and asks who called him, where "is the fool who spits on God?"