Word of God vs Good Book

Word of God and Good Book both are nouns.

Word	Noun	Adjective	Verb	Adverb
Word of God	Yes	No	No	No
Good Book	Yes	No	No	No

As nouns, Word of God and Good Book are synonyms defined as:

Word of God and Good Book: the sacred writings of the Christian religions

Other synonyms of Word of God include Bible, Book, Christian Bible, Holy Scripture, Holy Writ, Scripture, Word.

Word of God (noun)	Good Book (noun)
the sacred writings of the Christian religions	the sacred writings of the Christian religions
the message of the Gospel of Christ
a manifestation of the mind and will of God