Why we will never be part of the International Buddhist Council

We would have to accept the Three Marks of Existence as required by them, namely, impermanence (anicca), unsatisfactoriness or suffering (dukkha), and non-self (anattā). We refuse to accept a partial truth. The Three Marks of existence merely refer to the state of Temporary Existence, one of the Three Truths. The other two truths are non-substantiality and the Middle Way. The Middle Way is characterized by eternity, purity, joy, and true self.

For a further discussion of the Three Truths see the SGI dictionary:

https://www.nichirenlibrary.org/en/dic/Content/T/183