" have the faith of a Breton peasant's wife"?

Yes, devout and unthinking.

But for Pasteur, when he finally knows all the knowledge about the Universe, would he be such an unthinking creature?

Pasteur's faith was as genuine as his science. In his panegyric of Littré, whose fauteuil he took, he said: "Happy the man who bears within him a divinity, an ideal of beauty and obeys it; an ideal of art, an ideal of science, an ideal of country, an ideal of the virtues of the Gospel". These words are graven above his tomb in the Institut Pasteur. In his address Pasteur said further "These are the living springs of great thoughts and great, actions. Everything grows clear in the reflections from the Infinite". Some of his letters to his children breathe profound simple piety. He declared "The more I know, the more nearly is my faith that of the Breton peasant. Could I but know all I would have the faith of a Breton peasant woman." What he could not above all understand is the failure of scientists to recognize the demonstration of the existence of the Creator that there is in the world around us. He died with his rosary in his hand, after listening to the Life of St. Vincent de Paul which he had asked to have read to him, because he thought that his work like that of St. Vincent would do much to save suffering children.

Catholic Encyclopedia (1913) ?
Where did you find this?