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 Big Bang as a tool of the 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. Recent discovery of the gravitational wave background may, however, help us test the Big Bang paradigm in a more concise manner than whenever before; GWB serves as a probe of the earliest eras in the history of the Universe, those that, from our usual point of view of the already classical CMB, are shielded by the last scattering surface.

According to BGV, if the average expansion rate is

then any spacetime where such a rate is positive,

is geodesically past-incomplete.

Hence the natural need of the moment when it all began.