I'd say that it was the lesser-known Borde–Guth–Vilenkin theorem, not even the discovery of background relic radiation, which provided the definitive supporting pillar for those who prefer scientific euhemerization of creationism.

BGV, in short words, states that any viable cosmological model of an expanding Universe, when rewound toward the past, must hit the wall of boundary, or a singularity, within a finite amount of proper time. On the other hand, it leaves open the possibility of a prior state (Penrose and Gurzadyan, rejoice) and/or a quantum origin.

Did the universe have a beginning?

If the average expansion rate is

then any spacetime where such a rate is positive,

is geodesically past-incomplete.