It is not a Belief.

This is the knowledge of Non-dualism.

Called Advaita Vedanta.

Most people will never understand this concept.

Maybe Only the students of Advaita Vedanta and Spinoza… people like Albert Einstein.

Advaita Vedanta (/ʌdˈvaɪtə vɛˈdɑːntə/; Sanskrit: अद्वैत वेदान्त, IAST: Advaita Vedānta) is a Hindu sādhanā, a path of spiritual discipline and experience, and the oldest extant tradition of the orthodox Hindu school Vedānta. The term Advaita (literally "non-secondness", but usually rendered as "nondualism", and often equated with monism refers to the idea that Brahman alone is ultimately real, while the transient phenomenal world is an illusory appearance (maya) of Brahman. In this view, (jiv)Ātman, the experiencing self, and Ātman-Brahman, the highest Self and Absolute Reality, is non-different. The jivatman or individual self is a mere reflection or limitation of singular Ātman in a multitude of apparent individual bodies.

Source: https://en.wikipedia.org/wiki/Advaita_Vedanta