Atma Jnana cannot be gained if we did not believe in Ishwar or existence of God, Brahman in Hinduism. And why would that be so?
Atma jnana means knowledge of our true inner self, the realization that we are primarily an atman, a spirit that manifested human form to cleanse self of dross impurities within.
The state of Atma jnana is reached indulging in spirituality detailed in Bhagavad Gita. In present times Atma jnana is not possible through path of bhakti yoga or karma yoga alone. Indulgence in path of jnana yoga as detailed in Bhagavad Gita is necessitated to realize our true inner self.
To realize we are primarily an atman, a soul we have to indulge in teachings of Lord Krishna in Bhagavad Gita. Furthermore we can also do it by demolishing our ego in totality. At any given moment of life one of the two is asserting, our ego or voice of atman come from within our heart. Guided by promptings of atman come from within we can complete our spiritual journey in totality.
Completion of spiritual journey means regaining our lost pure prime pristine primordial form, becoming a pure atman in the cosmic journey of life. The moment human beings gained enlightenment, the purified atman finally liberated self from cycle of birth and death forever.
To gain Atma jnana, to realize our true inner self indulgence in spirituality is an absolute must. We just cannot realize self, realize god indulging in religious practices, rituals. It is belief in Brahman, God Almighty that paves way for realizing our true inner self.
Belief in a personal Deity related to religion, religious practices that never led one towards god. But belief in Ishwar (pronounced Ishwar); the creator of entire cosmos landed a seeker in domain of God Almighty. Ishwar, Brahman is Nirguna (without an attribute) and also Nirakaar (without a form). Belief in Ishwar, Brahman and also world of atmans is an absolute must to travel spiritual path successfully.