What is the core belief of orthodox jews

The core belief was that t hey should follow in the ancient traditions of Judaism (the Torah) exactly.

God is Trinity: One and Three simultaneously,

Jesus Christ is the second Person of the Holy Trinity,

God the Son, Incarnate, that is, both completely God and completely human.

The belief of the Orthodox Jews are mainly based on the 13 principles of Judaism. Orthodox Judaism is classified as monotheistic, meaning, they only believe and worship only God. Their core belief is that, they are the chosen people of God or which they call Hashem. Hope this answers your question.