In Chapter 9 of Book the Third, Sydney Carton walks to the chemists and as he does so, he recalls his conversation with the wood-sawyer and the man's proud answer that many were killed this day. Carton recalls the Bible verse as he thinks of these deaths, future deaths, the death of his beloved father, and his own end.
In this particular chapter, Carton has decided to take drastic action to save Charles Darnay, and this is an action which can redeem his own dissolute life. Up until now Carton has allowed Stryver to take credit for his genius, and he has demonstrated no redeeming qualities in his character. During his visit to the Carton home in Chapter 13 of Book the Second, Sydney deprecates himself before Lucie, but he declares his undying devotion to Lucie. Futhermore, because of Lucie's encouragement, Carton begins to believe that he might actually be able to redeem himself.
This possibility of self-redemption and the redemption of the martyrs is a key reason that Sydney Carton repeats the phrase "I am the resurrection...." Also, he connects this passage from the Bible as a reminder to himself of his own father's death, and he may wish to redeem himself to honor his father's name. Another reason is his sympathetic consideration of the poor souls killed by the guillotine. Finally, his love for Lucie, which gives some meaning to his dissipated life, motivates him to consider a sacrifice for her that would redeem his soul and leave her with good memories of him.
Since Carton wants to save Lucie's father and he cannot find any way to do it save ways that end with his own death, he is preparing to sacrifice himself in order to save the others. Just as he imagines Christ died in order to save all mankind, he is telling himself that he can do the same in order to save the family that has put their faith in him.
Much of the discussion and imagery in the chapter center around death and funerals and resurrection and Carton finds some solace in this even as he makes plans for his own death. It is not until Chapter 13 that his plot is fully revealed but it is here in chapter 9 that the basic outline of it begins to take shape, particularly the part that involves him sacrificing himself.