Bhagavad Gita is considered the best guide not only for confused souls but anyone in general. Its relevance is not just limited to Hindus, as is the perception. The message given by Lord Krishna to Arjuna before the great battle of Kurukshetra is applicable to any human being who is frightened, hesitant or unsure about carrying out his or her duty.
However, the message of the book is considered unacceptable to athiests or people who don’t believe in rebirth. For the uninitiated, Gita focuses a lot on aatma (soul) and preaches about how it leaves one’s body at the time of death and enters another body. Hence, the person takes rebirth or his or her reincarnation takes place.
Gita also tells us that we continuously keep taking birth after death. As we cast off old clothes and buy new ones, similarly our aatma too casts off the old body and goes into the body of a newly born baby.
So, obviously, it is impossible for a person not believing in rebirth to relate with this. But if you look closely, there is a verse which also gives you an option to not believe in rebirth or the theory of soul casting off the old body and going into a new one.
This is seen in Chapter 2, Verse 26 Krishna. Going into slight background, Krishna explains in this chapter how the soul never dies and just passes from one body to another. But in between, He says, “If, however, you think that the soul is perpetually born and always dies, still you have no reason to lament, O mighty-armed.”
This can be interpreted that even if you don’t believe in the theory of soul and rebirth, you should still not lament anybody’s death because the one who has taken birth has to die someday.
All this was in encouragement to Arjuna to pick up his weapons and fight the war. But it can also be applicable for anyone who is unsure or scared to fight his or her personal war, which can be any given challenge.
And with the aforementioned verse, it is hoped that the message even encourages non-believers. All it says is that there is no reason for us to not fulfil our duties.
P.S: While searching on the net, I found this point to be very well explained on a website, which you can read HERE.