Moses spoke of himself in order to lend likelihood to his prediction, and
not as a prophet by revelation, because in verse 21 of the same chapter we
are told that God revealed the same thing to Moses in different words, and
there was no need to make Moses certain by argument of God's prediction and
decree; it was only necessary that it should be vividly impressed on
his imagination, and this could not be better accomplished than by
imagining the existing contumacy of the people, of which he had had frequent
experience, as likely to extend into the future.

(18) All the arguments employed by Moses in the five books are to be
understood in a similar manner; they are not drawn from the armoury of
reason, but are merely, modes of expression calculated to instil with
efficacy, and present vividly to the imagination the commands of God.
(19) However, I do not wish absolutely to deny that the prophets ever argued
from revelation; I only maintain that the prophets made more legitimate use
of argument in proportion as their knowledge approached more nearly to
ordinary knowledge, and by this we know that they possessed a knowledge
above the ordinary, inasmuch as they proclaimed absolute dogmas,
decrees, or judgments. (20) Thus Moses, the chief of the prophets, never
used legitimate argument, and, on the other hand, the long deductions and
arguments of Paul, such as we find in the Epistle to the Romans, are in
nowise written from supernatural revelation.

(21) The modes of expression and discourse adopted by the Apostles in the
Epistles, show very clearly that the latter were not written by revelation
and Divine command, but merely by the natural powers and judgment of the
authors. (22) They consist in brotherly admonitions and courteous
expressions such as would never be employed in prophecy, as for instance,
Paul's excuse in Romans xv:15, "I have written the more boldly unto you in
some sort, my brethren."