Who and when first declared the Bible to be "The Word of God" ?

Probably .... in the middle of the fourth Century ... because the scriptures were not written until then. This is what the Church doesn't want you to know. Gosh all those people before then went to hell, because god had not written it before then? But the Bible was altered continually since and is being altered to this very day. Its the greatest story ever SOLD.

Jesus and writers of the Christian Greek Scriptures referred to the collection of sacred writings as “the Scriptures,” or “the holy Scriptures,” “the holy writings.” (Mt 21:42; Mr 14:49; Lu 24:32; Joh 5:39; Ac 18:24; Ro 1:2; 15:4; 2Ti 3:15, 16) The collection is the written expression of a communicating God, the Word of God, and this is acknowledged in phrases such as “expression of Jehovah’s mouth” (De 8:3), “sayings of Jehovah” (Jos 24:27), “commandments of Jehovah” (Ezr 7:11), “law of Jehovah,” “reminder of Jehovah,” “orders from Jehovah” (Ps 19:7, 8), “word of Jehovah” (Isa 38:4), ‘utterance of Jehovah’ (Mt 4:4), “Jehovah’s word” (1Th 4:15). Repeatedly these writings are spoken of as “sacred pronouncements of God.”—Ro 3:2; Ac 7:38; Heb 5:12; 1Pe 4:11.

Which "bible"? Every sacred text was declared to be the word of God - the earliest is lost in the mists of pre-history.