"The heart is deceitful above all things, and desperately wicked: who can know it?" [Jeremiah 17:9].
Fundamentalist Christians believe the Bible to be the inerrant and infallible word of God. But who wrote the Bible ? The short answer is men wrote the Bible. "For the prophecy came not in old time by the will of man: But holy men of God spake as they were moved by the Holy Ghost". [II Peter 1:21].
So this is the situation. Prophecy came through men, the Scriptures were written by men, but according to the Bible man's heart is deceitful above all things and desperately wicked. So which is more probable, that a god should speak through a man, or that a man should lie and deceive and say a god spoke through him ? You do the math.