Christian groups, like Evangelicals, Southern Baptists, and others, accept the Bible as the literal Word of God. This view is often summed up as:
“God said it, I believe it, that settles it”.
In other words, any passage in the Bible should be taken as if the words proceeded directly from God’s mouth. They don’t all agree on how words from God got written into the Bible. Some theorize that God dictated the text word-for-word to the various authors of the Bible and the authors faithfully recorded the words.
Other theories, like Verbal, Plenary Inspiration, are a bit more nebulous in that no one is really willing to say exactly what they mean by “inspiration”. Nevertheless, this theory also holds that every word in the Bible proceeds (somehow) directly from God. The Christian Faith website presents a succinct summary this belief.
These same Christian groups claim that the Gospels are first-hand eyewitness accounts. They believe that Matthew, Mark, Luke, and John are reporting their own first-hand, eyewitness accounts of their encounters with Jesus. They are reporting what they’ve seen with their own eyes and heard with their own ears. Since eyewitness accounts virtually always vary from person to person, the accounts in the different Gospels are different as we should expect. These accounts, though different and at times conflicting, are nevertheless compelling evidence of the historicity and the nature of Jesus. You and I should believe that Jesus is the Son of God based in part on these eyewitness accounts.
An eyewitness account by its very definition cannot be the Word of God.
If the words in the eyewitness accounts come from God, then the words are not the evangelist’s own eyewitness account. If the words are the evangelists own personal account, then the words are not God’s words. The two claims are incompatible: either all words in the Bible are from God (and there are no eyewitness accounts from the Evangelists), or the Bible contains eyewitness accounts which are not God’s words.
Of course it is also possible that words in the Bible do not come from God and that the Gospels are not eyewitness accounts. Those two claims are perfectly compatible.