In general, medieval Christianity focused more on the next world than on our happiness within this one. True happiness would only be found in Heaven, not on earth, and entrance to Heaven could only be found through religious piety. Free will was emphasized by St. Augustine (354–430 C.E.), the most influential Christian theologian in the first millennium C.E. Every individual has the free will to choose whether or not to follow God. Sex and the passions of the body were considered sinful, unless performed within a marriage for the purpose of childbearing. Belief in the devil was also widespread and mental illness was often seen as a result of possession by the devil.