By far, the main person responsible for making hell eternal in the Western Church was St. Augustine (354–430 CE). Augustine…was made Bishop of Hippo in North Africa. He did not know Greek, had tried to study it, but stated that he hated it. Sadly, it is his misunderstanding of Greek that cemented the concept of eternal hell in the Western Church. Augustine not only said that hell was eternal for the wicked, but also for anyone who wasn’t a Christian. So complete was his concept of God’s exclusion of non-Christians that he considered un-baptized babies as damned. When these babies died, Augustine softened slightly to declare that they would be sent to the “upper level” of hell. Augustine is also the inventor of the concept of “hell Lite,” also known as Purgatory, which he developed to accommodate some of the universalist verses in the Bible. Augustine acknowledged the Universalists, whom he called “tender-hearted,” and included them among the “orthodox.”
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Augustine
Source: Dr Ken Vincent – Brazen Church