All bright students and scholars like Abdul Kalam indulged in Bhagavad Gita never superficially but through path of contemplation (chintan) so as to reach the hidden underlying meaning! Once a seeker reaches the inner depth of spirituality contained in Bhagavad Gita, one becomes an absolute follower and believer of teachings of Lord Krishna in Bhagavad Gita.
It is always the initial experience of Bhagavad Gita that was truly exhilarating, beyond expectations. Henceforth the believer of Bhagavad Gita practically became a slave of spiritual teachings contained in Bhagavad Gita. This happened with Abdul Kalam and also Albert Einstein. The moment J. Robert Oppenheimer indulged in Bhagavad Gita he just could not believe the contents. He was totally mesmerized and floored by teachings of Lord Krishna relating to explanation of fundamentals of origin of cosmos.
Every single human being who was straightforward and logical in approach appreciated teachings of Bhagavad Gita! The first experience of Bhagavad Gita opened a plethora of expectations.
To understand fundamentals of every single aspect of cosmic life, Abdul Kalam started his indulgence in Bhagavad Gita as and when opportunity arose. A logical thinker can never reject doctrine of Bhagavad Gita. It was just not possible. Every single scientist, physicist, psychologist… quantum theory expert and astrophysicist who indulged in Bhagavad Gita even for once became an absolute fan of teachings of Lord Krishna in Bhagavad Gita.
Bhagavad Gita contained everything from origin of cosmos to dissolution of cosmos. Bhagavad Gita also explained why human beings need indulging in spirituality to reach cosmic end of life.
The teachings of Bhagavad Gita do not relate to any religion, they had universal appeal. Abdul Kalam though a Muslim was never bonded to Muslim religion. He was a logical thinker and believed in all aspects of spirituality contained in Bhagavad Gita.