During most of Lemaître's tenure in the academy, Pope Pius XII occupied the Chair of Peter. The Pope delivered his famous speech, "Un'Ora," after he analyzed Lemaître's science with the intent of developing a philosophical argument that one could ultimately use to prove the existence of God. This event immediately stimulated theological and scientific debate on the relationship of science and religion. Pius XII had provided two arguments relying on science to confirm philosophical positions that included God. First, he mentioned the instability of the universe. Pius XII thought it was logical that an immutable being had to have created the mutable physical world. Lemaître was not adamantly opposed to this line of reasoning. However, Pius' second idea was not as well received. The Pope said that the apparent organization that characterizes the entire universe was another indication. It appears that Pius XII's underlying assumption was that the supernatural act of divine creation began with the early stages described by the primeval atom hypothesis:
. . . contemporary science, with one sweep back across the centuries, has succeeded in bearing witness to the august instant of the primordial Fiat Lux,which along with the matter there burst forth from nothing a sea of light and radiation . . . Thus, with that concreteness which is characteristic of physical proofs, modern science has confirmed the contingency of the universe and also the well-founded deduction to the epoch when the world came forth from the hands of the creator.
Statements such as these contradicted Lemaître's own strict distinction between the tools for investigating matters of science and matters of theology. "He realized quite fully the tentative and hypothetical character of scientific theories and for this reason alone, if for no others, opposed the use of such theories to support philosophical, theological or faith statements." As a result, Professor Lemaître wanted his scientific theories to be judged exclusively on their physical merit, keeping metaphysical implications completely separate.