The First Vatican Council, Dei Filius (1870) Dogmatic Constitution on the Catholic Faith defined the following:
If anyone shall have said that the one true God, our Creator and our Lord, cannot be known with certitude by those things which have been made, by the light of human reason: let him be anathema.
From what I understand, this pronouncement was motivated at the time by those who were denying natural theology.
Can someone elaborate on some of the specifics regarding those who were denying "natural theology" at the time that Pope Bl. Pius IX made this (de fide) declaration?