Lemaitre, a Belgian Diocesan priest and also a scientist was a pioneer in applying GR to cosmology. He derived Hubbles law two years before Hubble actually experimentally demonstrated it to be true.
He was also the first to give an estimate for Hubbles constant.
Moreover, Nature has reported that after a vote in 2018, the International Astronomical Union had proposed that Hubbles law be renamed the Lemaitre-Hubble law to pay tribute to Lemaitre.
Einstein was initially sceptical about Lemaitre's claims at the Solvay Conference and pointed out that Friedman had also come up with a similar solution a few years earlier (and which he had criticised but had later withdrawn).
Both Friedman and Lemaitre had proposed an expanding universe but he was the first to say that this explains the observable redshift. It was not a purely theoretical calculation but one with observable consequences.
It was also Lemaitre that first came up with the Big Bang hypothesis calling it the 'hypothesis of the primeval atom.'
The Wikipedia article on Lemaitre points out that his work is under-appreciated in the USA. It figures.