Titus Lucretius Carus (99 - 55 BC) was a Roman poet and philosopher who strongly advocated the existence of man on a natural scientific level. He argued against the afterlife and the immortal soul, mocking religion as simple superstition.
He was also among the first to suggest, at least in writing, the creation of the universe through the work of natural laws rather than a supreme being. He also seems to have had some bouts with mental issues with later writers suggesting that the great Cicero appended many of his works. His opinions obviously made him a target for later Christian writers.
De Rerum Natura
(On the Nature of Things)