The term "word of God" may mean a word spoken by God without ahuman mouthpiece. But in the New Testament it regularly means aword or a message that a human speaks on God's behalf. So, forexample, in Hebrews 13:7 it says, "Remember those who led you, whospoke the word of God to you; and considering the result of theirconduct, imitate their faith." So the "word of God" in Hebrews 4:12probably refers to the truth of God revealed in Scripture thathumans speak to each other with reliance on God's help tounderstand it and apply it.