Yes, a Catholic Priest was the first person to propose the theory of an expanding universe

It’s the event we now call The Big Bang, but Lemaître called it a “Cosmic Egg exploding at the moment of creation.”

Lemaître based his 1927 paper titled ‘A homogenous Universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae’ on Einstein’s theory of General Relativity, proposing an expanding universe as opposed to a stable one. The paper was published in an obscure journal, seldom read outside of Belgium, perhaps part of the reason the theory is now known as Hubble’s Law, after Edwin Hubble who observationally confirmed Lemaître’s theory.

Georges Lemaître and Albert Einstein, 1933

It wasn’t until three years later that Lemaître suggested that the universe has been expanding from one particular point, a single particle, which would become known as the Big Bang theory.

The guy who called it The Big Bang Theory was making fun of Lemaître

It was Fred Hoyle, an astronomer, who named Lemaître’s theory the “big bang”. At the time of Lemaître’s single particle suggestion, physicists were sceptical of a “beginning of the universe” proposed by a Catholic priest.

Albert Einstein called Lemaitre a friend, but initially rubbished him

Einstein initially called Lemaitre’s physics “abominable”, having read his 1927 paper, suggesting Lemaitre’s calculations were correct but disagreeing with his conclusions about the expanding universe. But after hearing Lemaitre present at a series of seminars in California in 1933, Einstein admitted, “This is the most beautiful and satisfactory explanation of creation to which I ever listened”.

A caption in the New York Times Magazine, showing a photo of Einstein and Lemaître together at the Californian seminars read, “They have a profound respect and admiration for each other.”

Lemaitre spoke out against Pope Pius XII’s enthusiasm for The Big Bang

In 1951, Pope Pius XII gave a speech that made headlines, inferring that the Big Bang theory gave support to the idea of a universe created by God. For Lemaître, that was one step too far. He spoke out against the Pope to implore his theory’s neutrality on the God question, saying, “As far as I see, such a theory remains entirely outside any metaphysical or religious question. It leaves the materialist free to deny any transcendental Being.”