Constant was the son of a shoemaker in Paris. In 1832 he entered the seminary of Saint Sulpice to study to enter the Roman Catholic priesthood, as a sub-deacon he was responsible for catechism, later he was ordained a deacon, remaining a cleric to the rest of his life. One week before being ordained to the priesthood, he decided to leave the priestly path, however the spirit of charity and the life he had in the seminary stayed with him through the rest of his life, later he wrote that he had acquired an understanding of faith and science without conflicts.