Which feature o
f the enlightenment had a profound effect on the rise of democratic ideas?

Enligtenment emphasized a lot a rationality of human beings. Human reason started to be seen as a basic characteristic of humans. Whereas in the past irrational, divine, mystical aspect was always more or less present, Enlightenment (probably because of increasing secularization) stressed reason not only in humans but also in the nature. All that became also basis of envisioning democratic regime (general right to vote was practiced already in 1789, at the very beginning of the French revolution). Modern democracy started to be built on the idea of rationality of humans.

Explanation: Of course, belief in human reason was exaggerated and onesided because human nature is much more complex. It was already romanticism that put emphasis on the other side romanticism revealed that humans are not 100% rational (already Kant proved that "reason" has its limits). Of course, some were shocked by killing during the French revolution and became aware of irrationality of humans (for ex. Friedrich Schiller).