The angel is very clear in this verse about the effect of the loss of the plain and precious parts. An apostasy occurs. Men stumble. Satan has power over them. Why is this related to the removal of "plain and precious things" from the book? Because the book is a symbol of the true gospel. Of course the true gospel could never lead to these consequences. From a symbolic standpoint, only the corruption of the source is sufficient explanation, as the source could not be bad. Thus in these verses the angel both confirms the truth of the gospel and describes the apostasy from it.