Whether Celtic Gaul was evangelised in the days of the Apostles or not, it is at least historically certain that as early as the middle of the second century the country was sufficiently provided with churches to form the principle theatre of the great persecution under Marcus Aurelius. And as Gaul was notoriously evangelised only piecemeal and slowly, and not by a sudden outburst of religious fervour, it is quite possible that the foundation of the Church of Chartres dates back from the first century. If this is so, it accords well with local traditions drawn from various sources, but agreeing in substance. For it is said[8] that S. Peter sent forth S. Savinian, S. Potentian and S. Albin to preach the Gospel to the Gallic nation. In the course of their mission they came to Sens and converted many virtuous heathen, amongst whom were Sérotin and Eodald. To these two, together with Potentian and Altin, Savinian, warned by a mysterious vision, gave this charge:--‘Take unto you,’ he said, ‘the shield of an unconquerable faith; go through the other towns of Gaul and banish all false superstition by preaching everywhere the truth of the Gospel.’ Leaving him, therefore, to organise the church at Sens, they set forth to Orléans, and thence, coming down that Roman road which is still known as the _Chemin de César_, they arrived at Chartres. There, perhaps, they found the altar of the Druids and the statue to the Virgin who should bear the Unknown God. Him declared they unto the people. As the word of the missionaries was supported by the miracles they wrought, and the saintly lives they led, a great part of the inhabitants believed on Christ. ‘When they saw this, the holy men of God consecrated a church dedicated to the glory of Mary, Mother of God, and _built just within the walls of the town_.’ The northern walls of Chartres, under the Romans, ran along the street _du Cheval Blanc_, where now the houses of the cloister stand. The site of the Church of S. Potentian may therefore be identified, as on other grounds we naturally should identify it, with that of the Chapel of Notre-Dame-de-Sous-Terre in the crypt of the Cathedral. To that impressive underchurch we will pay a visit at the end of this chapter, and endeavour to describe the various stages through which it has passed. But first we will trace the tragic history of these Christian pioneers.