How to understand secularism almost 150 years after the invention of the term? Why did this neologism contribute to instilling a democratic dynamic in the concrete implementation of the republican ideal in France, in particular through school? How can going back to the foundations of this political and social project make it possible both to understand a complex concept and to envisage its deployment today, by reinscribing the social question at the heart of the Republic? In the period we are living through, secularism gives the illusion, through the omnipresence of the term and despite multiple uses, of being a common reference. As if it sufficed to state the word to imagine that it is immediately understood. However, even if the concept has structured our mode of political organization for a long time, the fact remains that apart from a vague intuition which guarantees its value, it is far from always being understood.