There seems to be two elements at play here. The first is that clarity is needed on the prompt itself. From what I gather, the prompt is asking you to make a case as to how Dantes's transformation is akin to Dumas's motif of showing the hero as an instrument of a divine plan. In other words, one must examine how Dantes becomes representative of a divine construction of the good. The prompt is asking how Dantes's characterization in the novel represents Dumas's overall motif or idea that the hero is an extension of a divine plan.
From such an understanding, examining Dantes's growth and transformation throughout the novel becomes critical. The growth of Dante into being an instrument of the divine plan can be seen in how he views his imprisonment. Though he is imprisoned under unfair and unjust conditions, Dantes does not commit suicide or forsake any wider element. He endures and exists, and in doing so, recognizes that something larger is intended for him. When he escapes, he understands the need to embrace what it is to be an "extraordinary being." Dantes embraces the idea of a being an instrument in a divine plan when he says that “the most beautiful, noblest, most sublime thing in the world is to recompense and punish.” Dantes does not view himself as a regular person. He readily acknowledges himself as a tool of divine Providence. In this, Dantes believes that he acts in accordance to a divine plan. He rewards and helps those who have been good to him and punishes those who have violated the natural order of things by acting in a duplicitous and deceptive nature.
At the same time, Dantes acts an instrument of the divine plan. He sees himself as one who recognizes that there are two forms of sight: “There are two ways of seeing: with the body and with the soul. The body's sight can sometimes forget, but the soul remembers forever.” Dantes embraces the idea that his function is not merely to be that of the body, but rather of a larger and more collective soul. It is here in which Dantes's embodiment of the divine plan, and acting in accordance to the Dumas' motif, is apparent. This understanding helps Dantes acknowledge the limitations of pure vengeance: “Having reached the summit of his vengeance by a long and tortuous path, he saw an abyss of doubt on the other side of the mountain." Dantes recognizes the limitations of revenge, choosing instead to submit himself to a divine plan and being an instrument of one. He sees with the soul, and in doing so, readily acknowledges his condition as being an agent of the divine plan. It is for this reason that he seeks to not take more life, but rather do his duty and them progress to finding happiness away from a world of revenge. When Dantes acknowledges that "God is for and with me," it is his own internalization of the novel's closing words: "All human wisdom is contained in these two words, 'Wait and Hope." Through such a vision, Dantes understands what it means to fully submit to a divine plan and he sees himself as being its instrument.