Septimus was a Roman of the planet 892-IV, and the de facto patriarch of a band of fugitive slaves practicing a forbidden religion in a world where the Roman Empire remained in power through the beginnings of the modern era. Septimus' variation of Christianity, the Children of the Son, was only recently on the rise, or in resurgence.
Formerly an Imperial senator practicing the state religion of polytheistic-consumerism, Septimus converted to Son-worship, resulting in his expulsion from the Senate and sale into slavery. Escaping from servitude, Septimus spent his old age hunted by the Empire, living humbly in wilderness caverns with his brethren. Guided by the precepts of his faith, Septimus expressed no bitterness concerning his state. His serenity and patience served as an example that younger Son-worshipers, such as former gladiator Flavius Maximus, strove to follow.
In 2268 three travelers of surpassing strangeness (James T. Kirk and company) appeared in the wilderness, and were brought before Septimus for disposition. Over the objections of the suspicious Flavius, Septimus allowed the strangers a chance to prove their peaceful intent, and offered them hospitality. The USS Enterprise landing party's mission would put anyone offering aid in direct conflict with the parallel Rome. In spite of the increased risks to the band's already perilous existence, and ignoring Kirk's dismissive condescension toward his faith, Septimus agreed to help by supplying native clothing and the armed escort of Flavius.
Kirk, though grateful at the time, later expressed chagrin concerning his attitude toward Septimus and his brethren. After revealing his ignorance of basic knowledge of the physical universe, Septimus' exaltation of the Son was perceived as a primitive worship of the local star, rather than a nascent monotheism. The misunderstanding was enabled by the world's development of a parallel-English language, allowing the universal translator to go unused. The landing party was corrected later by communications officer Lieutenant Uhura, who monitored planetary broadcasts with enough care to realize Septimus' references were to the Son of God. (TOS: "Bread and Circuses").