The answer to your question can be divided into three main parts:

1.

The Formation, Divergence, and Decline of Conscience

The answer to your question can be divided into three main parts:

1. Formation of Conscience
Based on the materials, conscience begins forming in early childhood. Its initial state is characterized by immaturity, and it requires further upbringing and experience for its development. For example, one source states:
"Formation of the Christian conscience is carried out in a dynamic process in which a person's ability to see the meaning and value—or, on the contrary, the triviality and baseness—of their own actions in the light of Christian teachings on morality is perfected. In early childhood, in its initial state and manifestation, the conscience is still completely underdeveloped. It needs development and nurturing, gradually forming along with the other forces and capacities of the soul. This point suggests that the grace and action of the gifts of the Holy Spirit are vital in the process of forming the conscience. Without proper upbringing, the conscience may remain deaf and callous. Proper Christian nurturing of the conscience is achieved in the life of the Church."
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2. The Development of Conscience in Atheists and Believers
The materials indicate that approaches to the development of conscience differ significantly:

— For believers (in this case, Christians), the development of conscience is closely tied to religious education and the life of the Church. Here, traditional Christian virtues, such as mercy, meekness, patience, and love, play a key role in forming a firm foundation for moral choice and self-control.

— For atheists, according to one source, the process is determined by different principles. A critical view of atheism is described, suggesting that it deprives a person of a higher sense of responsibility and, consequently, turns morality into a mere declaration. The author writes:
"Atheism is not aesthetic. In the visible world, it neither feels nor contemplates the highest beauty. Atheism teaches: ‘Life itself is beautiful, reality itself is beautiful’; thus, Beethoven’s sonatas and a patient’s moans are equally beautiful: both are life. The patient’s moans are even more real, therefore, they must be more beautiful than an artificially created sound harmony.

But is the reality of a corpse being devoured by graveworms also beautiful?

Or the tragedy of death in all nature not illuminated by the light of eternity? Atheism cannot provide a criterion and category of beauty, because beauty, like morality, is an innate feeling of the human soul. By stripping away a person’s sense of responsibility before a higher truth—not only for actions but also for the feelings and thoughts of the soul—it destroys the roots of morality and turns it into a mere declaration."
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Thus, for believers, the development of conscience is associated with spiritual and moral education through religious values, whereas the atheist approach relies on understanding morality through empirical and social mechanisms, which, in the author’s opinion, can lead to the dilution of genuine moral orientation.

3. The Loss of Conscience
Regarding the loss of conscience, the sources indicate that its sharpness may diminish under certain circumstances related to systematic violations of one’s internal moral compass, when a person consciously or subconsciously refuses to heed the reproaches of their inner voice. One text describes this process in detail:
"The matter of conscience, as a lawgiver, is to show the laws by which a rational, free being should operate and to incline its will by the force of its obligation (56, p. 267). … The burning by conscience is an inner affair… The conscience itself never burns out; however, the soul may reach such numbness to its prompts and reproaches that one might as well say the conscience is gone, burned out. A burnt conscience feels nothing… sins with full awareness, and suffers little."
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Furthermore, another source notes that with systematic violations, when a person refuses to work on themselves and permits indifference to moral norms, the conscience loses its clarity, which is reflected in the quality of the decisions made:
"Conscience guides a person in making faultless moral decisions. However, under certain conditions, such as systematic violations, a person may lose the keenness of their conscience, leading to actions far from the truth. Only sincere work on oneself and the desire for self-improvement can restore it."
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Thus, the loss of conscience does not occur instantly, but as a result of a prolonged process during which a person loses the sense of inner responsibility and the ability to perceive moral reproach for their actions.

Supporting citation(s):
"Formation of the Christian conscience is carried out in a dynamic process, ... In early childhood ... it needs development and nurturing ..." (source: link txt, page: 485)
"Atheism is not aesthetic. ... By stripping away a person’s sense of responsibility before a higher truth ... it destroys the roots of morality ..." (source: link txt, page: 767)
"The matter of conscience, as a lawgiver, ... A burnt conscience feels nothing..." (source: link txt, page: 367)
"Conscience guides a person in making faultless moral decisions. However, under certain conditions, ... only sincere work on oneself ... can restore it." (source: link txt, page: 31)