Members of a learned class of priests, teachers, and judges among the ancient Celtic peoples, the druids instructed young men, oversaw sacrifices, judged quarrels, and decreed penalties. They did not engage in warfare and paid no tribute. They studied ancient verse, natural philosophy, astronomy, and religious lore; their principal doctrine was belief in the immortality of the soul and the belief that the soul passed into another body after death. They sometimes practiced human sacrifice to cure gravely ill people or protect warriors in battle. The druids were suppressed in Gaul by the Romans in the first century ce and in Britain a little later. After Christianity came to Ireland, they lost their priestly functions, but survived as poets, historians, and judges.