Every major world religion puts forward some unique metaphysical idea which distinguishes it.

Christianity: (1) God is a triune being (2) God "the Son" incarnated into humanity to offer himself as sacrifice to atone for sins of mankind (3) sin has to be atoned through a blood sacrifice of someone absolutely pure

For them God is not absolutely One and indivisible. He exists with distinct personages each possessing their own will etc.

For me this is definitely the greatest theological problem of Christianity

Hinduism: Reincarnation and the caste system - The caste you are born into is part of your "cycle" of reincarnation. So those of lower caste are inferior to high caste in terms of being closer to Moksha (liberation from the cycle of rebirth)

There can be no denying that the great social inequality and discrimination in India was based on orthodox Hindu philosophy which gave religious sanction to an apartheid system!

Sikhism: Personification of a book (Guru Granth Sahib)

For Sikhs, the Granth is an actual person. Like the Christian "mystery" of the metaphysical change of the bread into flesh and wine into blood, the Sikhs have their own metaphysical "mystery" that is nonetheless irrational, regarding the Granth being an actual person that must be treated as a human Guru!

The 5 Ks which baptised "Amritdhari" Sikhs must observe reveals Sikhism as a martial order rather than an actual complete Religion

Buddhism: The 4 "Noble Truths", that life is suffering and that the way to be free from suffering is through detachment. A very nihilistic and cynical view of human life and human condition that is colorless and unemotional. Buddhism basically wants to strip humans of their humanity and become unfeeling and detached and apathetic.

Mormonism: Polytheism - that there are many gods in the cosmos and the doctrine of "Exaltation" that God was once a man and that man can become as God is

Baha'i Faith: The "New World Order" of the Bahai faith makes it clear that this religion is not a religion at all but actually a political ideological movement that seeks to create a one-world government with a single language and religion for everyone.