In the year 400 he wrote two books _Against the Party of Donatus_, which are also lost; and about the same time he published his refutation of the letter of Parmenianus in answer to Tichonius, in which he handles and solves the famous question, whether, while abiding in unity in the communion of the same sacraments, the wicked pollute the good by their society.[3] Then followed his seven books _On Baptism_, included in this volume, in which he shows the emptiness of the arguments of the Donatists for the repetition of baptism; and proves that so far was Cyprian from being on their side, that his letters and conduct are of the highest value as overthrowing their position, and utterly condemning their separation from the Church. Not long after this, Petilianus, bishop of Cirta or Constantina, the most eminent theologian among the Donatist divines, wrote a letter to his clergy against the Catholics, of which Augustine managed to obtain a copy, though the Donatists used their utmost care to keep it from him; and he replied to it in two books, written at different times,--the first in the year 400, before he was in possession of the whole letter, the remainder in 402. To the first book Petilianus made an answer, of which we gather the main tenour from a third book written by Augustine in reply to it. It appears to have been full of vehement abuse, and to have assumed the question in dispute, that the existence of the true Church, and the catholicity of any branch of it, depended on the purity and orthodoxy of all its ministers; so that the guilt or heresy of any minister would invalidate the whole of his ministerial acts. Hence he argued that Cæcilianus being the spiritual father of the so-called Catholics, and having been a _traditor_, none of them could possibly have been lawfully baptized, much less rightfully ordained. Augustine admits neither of his assumptions; but, leaving the guilt or innocence of Cæcilianus as a point which was irrelevant (though practically the case against him utterly broke down), he addresses himself to the other point, and argues most conclusively that all the functions of the clergy in celebrating the rites of the Church being purely ministerial, the efficacy of those rites could in no way depend upon the excellence of the individual minister, but was derived entirely from Christ. Hence there was a certainty of the grace bestowed through the several ordinances, which otherwise there could not possibly have been, had their virtue depended on the character of any man, in whom even an unblemished reputation might have been the fruit of a skilled hypocrisy.