Celsus, therefore, not investigating in a spirit of impartiality the facts, which are related by the Egyptians in one way, and by the Hebrews in another, but being bewitched, as it were, in favour of the former, accepted as true the statements of those who had oppressed the strangers, and declared that the Hebrews, who had been unjustly treated, had departed from Egypt after revolting against the Egyptians,— not observing how impossible it was for so great a multitude of rebellious Egyptians to become a nation, which, dating its origin from the said revolt, should change its language at the time of its rebellion, so that those who up to that time made use of the Egyptian tongue, should completely adopt, all at once, the language of the Hebrews!
Let it be granted, however, according to his supposition, that on abandoning Egypt they did conceive a hatred also of their mother tongue, how did it happen that after so doing they did not rather adopt the Syrian or Phœnician language, instead of preferring the Hebrew, which is different from both? But reason seems to me to demonstrate that the statement is false, which makes those who were Egyptians by race to have revolted against Egyptians, and to have left the country, and to have proceeded to Palestine, and occupied the land now called Judea. For Hebrew was the language of their fathers before their descent into Egypt; and the Hebrew letters, employed by Moses in writing those five books which are deemed sacred by the Jews, were different from those of the Egyptians.
Source: Against Celsus (New Advent)