The earliest stage of Christianization of the various Celtic people and Germanic people occurred only in the western part of Germany, the part controlled by the Roman empire. Christianization was facilitated by the prestige of the Christian Roman Empire amongst its pagan subjects and was achieved gradually by various means. The rise of Germanic Christianity was at times voluntary, particularly among groups associated with the Roman Empire. Aspects of primeval pagan religion have persisted to this day, including the names of the days of the week.