This _sola-gratia_-doctrine, the vital truth of Christianity, rediscovered and proclaimed once more by Luther, was, as stated, the target at which Erasmus directed his shafts. In his _Diatribe_ he defined the power of free will to be the faculty of applying oneself to grace (_facultas applicandi se ad gratiam_), and declared that those are the best theologians who, while ascribing as much as possible to the grace of God, do not eliminate this human factor. He wrote: Free will is "the ability of the human will according to which man is able either to turn himself to what leads to eternal salvation or to turn away from it." (St.L. 18, 1612.) Again: "Those, therefore, who are farthest apart from the views of Pelagius ascribe to grace the most, but to free will almost nothing; yet they do not abolish it entirely. They say that man cannot will anything good without special grace, cannot begin anything good, cannot continue in it, cannot complete anything without the chief thing, the constant help of divine grace. This opinion seems to be pretty probable because it leaves to man a striving and an effort, and yet does not admit that he is to ascribe even the least to his own powers." (1619.) One must avoid extremes, and seek the middle of the road, said Erasmus. Pelagius had fallen into Scylla, and Luther into Charybdis. "I am pleased with the opinion of those who ascribe to free will something, but to grace by far the most." (1666.) Essentially, this was the error held, nursed, and defended also by the Synergists, though frequently in more guarded and ambiguous phrases. But their theory of conversion also involved, as Schaff and Schmauk put it, "the idea of a partnership between God and man, and a corresponding division of work and merit." (_Conf. Principle,_ 600.)