The argument which Celsus employs against us and the Jews will be turned against himself thus: My good sir, does the God who is over all things know what takes place among men, or does He not know? Now if you admit the existence of a God and of providence, as your treatise indicates, He must of necessity know. And if He does know, why does He not make (men) better? Is it obligatory, then, on us to defend God's procedure in not making men better, although He knows their state, but not equally binding on you, who do not distinctly show by your treatise that you are an Epicurean, but pretend to recognise a providence, to explain why God, although knowing all that takes place among men, does not make them better, nor by divine power liberate all men from evil?
We are not ashamed, however, to say that God is constantly sending (instructors) in order to make men better; for there are to be found among men reasons given by God which exhort them to enter on a better life. But there are many diversities among those who serve God, and they are few in number who are perfect and pure ambassadors of the truth, and who produce a complete reformation, as did Moses and the prophets. But above all these, great was the reformation effected by Jesus, who desired to heal not only those who lived in one corner of the world, but as far as in Him lay, men in every country, for He came as the Saviour of all men.
Source: Against Celsus (New Advent)