This is a great question. Descartes was born on March 31, 1596 he died February 11, 1650. This fact is significant, because he lived after the Protestant Reformation and the Renaissance. During these two times, intellectuals questioned everything. In particular, they questioned tradition and religious authority. The irony, of course, is that the Protestant Reformation was a religious movement that questioned religion.
The Reformation provided the key to question all knowledge. In light of this, Descartes lived in a time, where it was possible to question knowledge itself. He did so, but as he did so he did it as a pious Catholic, at least according to his own confession. Others disagreed, like his contemporary Pascal.
Now did he ever gain knowledge himself? Yes and no. No one can exhaust knowledge. Moreover, he is considered the father of modern philosophy. Hence, he was just the beginning. So, his work is significant, but just the first step.