I have read, a highly revered mystic and ascetic of Hinduism -- Lord Ramakrishna, the Guru of Swami Vivekananda, had a worldview that every religion is true and every religion eventually leads to God. They differ in practices and methods, but they ultimately lead to the same goal.
My question is, does the above worldview factually true? I have interacted with Muslim friends who says the way the above mentioned ascetic practised countless number of religions including Islam isn't simply acceptable to Islam because once somebody takes Islam they cannot literally continue any form of idolatry or polytheism any more. If the ascetic deviates even a trace from the Islamic commandments or literature, then he is no more practising Islam.
Similarly, Buddhism does not believe in god. It looks for Nirvana, that is total annihilation of the soul. (Please correct me if my understanding is wrong.)
Although religions superficially gives similar advise like don't tell lie, don't steal others' stuff etc, the philosophical principles or worldviews practised by them are drastically different.
So my question is; if there is really any unifying principle between all religions or they are fundamentally different?