An argument that originated in Greek philosophy and was adopted by Christian theologians, First Cause states that everything is caused by something, and if traced back far enough, God was the first thing to cause the existence of another thing. However, asking who or what caused God to come into being brings that theory into question. Some would answer that God is self-caused, but to Russell, this argument is inherently flawed and easy to disprove.

The Latin term for “let it be done,” fiat originally stems from Mary consenting to carry God’s child. In addition, it signifies Jesus’s obedience to God. In the context of “Why I Am Not a Christian,” it refers to God’s will. If one were to say that someone rules by fiat, this might not be considered a compliment but rather a comment on how controlling, demanding, or even tyrannical one’s rule might be. It’s hard to ignore that definition in the context of Russell’s lecture. However, he’s directly referencing God’s fiat as God’s will—that is, what God wishes to be done shall be done.