Q: Gita says Krishna is all powerful God. Then how is Gita relevant for those who do not believe so? How is believing Krishna to be all powerful God different from believing Muhammad to be Prophet or Jesus to be son of God?
If you study basics of Vedas and Yog Darshan before studying Gita, you would realize that Gita nowhere demands anyone to believe in Krishna as all powerful God. Gita is a message to inspire a confused Arjun into path of relevant action.
The shlokas where Krishna talks in first person about glory of God form foundation of Hinduism – that God is within you. You need not seek him above seventh sky.
Instead if you follow path of Yog, you will break all barriers of ego and see only Him.
This God within you is all powerful, source of everything, and the Only One worth being pursued.
Depending upon your spiritual level and approach, you can adopt various means to seek this God. That is a matter of personal faith.
Gita is an elaboration of 40th chapter of Yajurveda and summary of spiritual wisdom of entire Vedas. It is for everyone who has an open mind, urge to live worthwhile life, and is committed to not be a lazy escapist.
(More details in translation of ‘The Original Gita’, whenever that is completed and released in public)