And I am sure that the king of Egypt. God forearms his people, lest, suffering a repulse at their first onset, they should retire, and abandon in despair the work enjoined to them. It was, indeed, a hard thing to hear that their expedition would be vain; and that they might as well address themselves to the trunk of a tree, since there was no hope of reaching the obstinate heart of Pharaoh; but they would have been much more discouraged by this trial, if his stubbornness had been discovered unexpectedly. Therefore God foretells that their words would avail nothing; but at the same time he announces that he should succeed by his own wondrous power. If any think it absurd for these unhappy men to be wearied by their useless labor, and to be repulsed with ridicule and insult, I answer, that this was for the sake of example, and that it was advantageous for setting forth God's glory, that the king, having been civilly applied to, should betray his impious perversity, since nothing could be more just than that what he had unjustly refused, should be extorted from him against his will. But interpreters differ as to the meaning of the words. For some translate it literally from the Hebrew, "no, not by a mighty hand;" as though God said that the pride of the king would be unconquerable, and not to be subdued by any power or force; but the context requires a different sense, because the remedy is afterwards opposed to it, "and I will stretch out my hand;" and the result is added, that Pharaoh, overcome at length by the plagues, would let the people go. And this view is grammatically correct; for the Hebrews use the word vl', [1] velo, for "except." Therefore God commands his people to be firm and confident, although Pharaoh may not immediately obey; because he would evidence his power in a remarkable manner for their deliverance. In the meantime he arouses them to hope by the promise of a successful issue; since he will forcibly compel Pharaoh to yield.

Footnotes

1 - vl' is here rendered unless by the LXX., Vulgate, Pagninus, Luther, Vatablus, and Diodati; and by the equivalent, but in the margin of A V S.M. has neque; but adds, "alii exponunt vl' pro nisi." -- W

No, not - See the marginal rendering. Others explain it to mean, Pharaoh will not let the people go even when severely smitten.

I am sure that the king of Egypt will not let you go, no, not by a mighty hand - When the facts detailed in this history have been considered in connection with the assertion as it stands in our Bibles, the most palpable contradiction has appeared. That the king of Egypt did let them go, and that by a mighty hand, the book itself amply declares. We should therefore seek for another meaning of the original word. ולא velo, which generally means and not, has sometimes the meaning of if not, unless, except, etc.; and in Becke's Bible, 1549, it is thus translated: I am sure that the kyng of Egypt wyl not let you go, Except wyth a mighty hand. This import of the negative particle, which is noticed by Noldius, Hebrews. Part., p. 328, was perfectly understood by the Vulgate, where it is translated nisi, unless; and the Septuagint in their εαν μη, which is of the same import; and so also the Coptic. The meaning therefore is very plain: The king of Egypt, who now profits much by your servitude, will not let you go till he sees my hand stretched out, and he and his nation be smitten with ten plagues. Hence God immediately adds, Exodus 3:20 : I will stretch out my hand, and smite Egypt with all my wonders - and after that, he will let you go.

And I am sure that the king of Egypt will not let you go,.... Or "but" (c) "I am sure", &c. though so reasonable a request was made him, yet it would not be granted; this is observed to them, that they might not be discouraged when he should refuse to dismiss them, which the omniscient God knew beforehand, and acquaints them with it, that, when it came to pass, they might be induced to believe that the mission of Moses was of God, rather than the contrary:
no, not by a mighty hand; the mighty power of God displayed once and again, even in nine plagues inflicted on him, until the tenth and last came upon him; or "unless by a mighty hand" (d), even the almighty hand of God; prayers, entreaties, persuasions, and arguments, will signify nothing, unless the mighty power of God is exerted upon him.
(c) "ego autem", Junius & Tremellius, Piscator; "sed ego", V. L. (d) Sept. "nisi", V. L. Pagninus, Vatablus; so Noldius, p. 344. No. 1246.

I am sure he will not let you go - God sends his messengers to those whose obstinacy he foresees, that it may appear he would have them turn and live.