But the Hebrews, oppressed by the multiplied evils of slavery, directed their complaints to heaven, and cherished the hope of assistance from God. Then, as Moses was feeding his sheep, suddenly a bush appeared to him burning, but, what was surprising, the flames did it no harm. Astonished at such an extraordinary sight, he drew nearer to the bush, and immediately God spoke to him in words to this effect, that he was the Lord of Abraham, Isaac, and Jacob, and that he desired that their descendants, who were kept down under the tyranny of the Egyptians, should be delivered from their sufferings, and that he, therefore, should go to the king of Egypt, and present himself as a leader for restoring them to liberty.
When he hesitated, God strengthened him with power, and imparted to him the gift of working miracles. Thus Moses, going into Egypt, after he had first performed miracles in the presence of his own people, and having associated his brother Aaron with him, went to the king, declaring that he had been sent by God, and that he now told him in the words of God to let the Hebrew people go. But the king, affirming that he did not know the Lord, refused to obey the command addressed to him.
And when Moses, in proof that the orders he issued were from God, changed his rod into a serpent, and soon after convened all the water into blood, while he filled the whole land with frogs, as the Chaldæans were doing similar things, the king declared that the wonders performed by Moses were simply due to the arts of magic, and not to the power of God, until the land was covered with stinging insects brought over it, when the Chaldæans confessed that this was done by the divine majesty.
Then the king, constrained by his sufferings, called to him Moses and Aaron, and gave the people liberty to depart, provided that the calamity brought upon the kingdom were removed. But, after the suffering was put an end to, his mind, having no control over itself returned to its former state, and did not allow the Israelites to depart, as had been agreed upon. Finally, however, he was broken down and conquered by the ten plagues which were sent upon his person and his kingdom.
Source: Sacred History (New Advent)