Word of God
|For it is so written in|
The expression "Word of God" is used in two main senses.
 On the one hand...
...it refers to a (symbolic) identification of God with some linguistic expressions (logia) cf. John 1.1 "In the beginning was the Word, and the Word was with God, and the Word was God" or 1. Thessalonians 1 ""...our gospel did not come to you in word only, but also in power, and in the Holy Spirit and in much assurance...".
 On the other...
...it refers to the entirety of the Bible, and is used as a justification of the position that Biblical injunctions are the only possible basis for morals, ethics, and law. While the specific mysticism of the first sense (also found in Heracleitus and other non-scriptural sources) is still maintained in much Christian theology, it is only tangentially related to the second sense, which is our main concern here.
 Potential problems with the second
The idea that the Bible is the word of God presupposes several assumptions. First, that there is a (single, unique - or indeed any) God. Next, that this God uses expressions of ordinary (human) language to express its will, intentions, commands, etc. Finally, that a literal reading of this language is both necessary and sufficient for establishing moral/ethical/legal precepts. Here we take issue with all three assumptions, noting that (1) even strong belief in a (unique) supreme being (as seen in all monotheistic religions) is insufficient to establish the God of the Bible as this (unique) being, (2) an equally broad secular mystic tradition asserts that God uses mathematics to lay down the law, and (3) as with any writing of great antiquity, the philological and conceptual difficulties in establishing a literal reading are immense.
As for the first assumption, the greatest problem is that the same "word of God" argument can be used to justify both the Old Testament system, the New Testament system, the system of Islamic law, the system of Latter Day Saints rules and regulations, and so forth. To the extent that these system contradict each other in significant aspects, it is clear that they can't all be the same word of the same God. This, in spite of the contrary intention of those using this phrase as an argument in favor of their particular brand of orthodoxy, opens the door to moral relativism.
The second assumption, that God directly communicated his intentions to the humans who wrote down the Scriptures, looks hopelessly naive (even if we grant that the world is governed by eternal laws, and that such laws also exist for human affairs not just for physics, secondary assumptions that we will not analyze in greater detail here). The problem is not just that the idea is naive, for there may be naive ideas that turn out to be right, but rather that we lack evidence. To the extent there is evidence (and we can, for the sake of the argument, admit a broad class of evidence including fables and traditions) the same evidence of divine inspiration is also available for Buddha, Confucius, and other sages outside scriptural bounds. In this respect too, the "word of God" line of argumentation opens the door to moral relativism.
The final assumption (which usually takes the form that the King James Version of the Bible is all one needs to study) is contradicted by a mountain of scholarship, both religious and secular, that we need not detail here, as it is evident from comparing various (authorized) translations of the Bible. Note further that the natural growth of the halacha (and other biblically grounded forms of law over the millennia) proves that those interested in maintaining a biblical system of justice do not find the word of God sufficient as it is (for if it was sufficient there wouldn't be a need for elaboration), and of course it is highly debatable whether a religious (let alone Biblical) foundation of morals/ethics/law is necessary.