From henceforth Luther, while pursuing with unabated zeal his active duties at the university and in the pulpit at Wittenberg, and taking up his pen again and again to write short pamphlets of a simple and edifying kind, occupied himself untiringly with controversial writings, with the object partly of defending himself against attacks, partly of establishing on a firm basis the principles he had set forth, and of further investigating and making plain the way of true Christian knowledge. He first addressed himself to German Christendom, in German, in his 'Sermon on Indulgences and Grace.' His inward excitement is shown by the vehemence and ruggedness of expression which now and henceforth marked his polemical writings. It recalls to mind the tone then commonly met with not only among ordinary monks, but even in the controversies of theologians and learned men, and in which Luther's own opponents, especially that high Roman theologian, had set him the example. In Luther we see now, throughout his whole method of polemics, as we shall see still more later on, a mighty, Vulcanic, natural power breaking forth, but always regulated by the humblest devotion to the lofty mission that his conscience has imposed upon him. Even in his most vehement outbursts we never fail to catch the tender expressions of a Christian warmth and fervour of the heart, and a loftiness of language corresponding to the sacredness of the subject. In the midst of these labours and controversies, Luther had to undertake a journey in the spring of 1518 (about the middle of April) to a chapter general of his Order at Heidelberg, where, according to the rules, a new Vicar was chosen after a triennial term of office. His friends feared the snares that his enemies might have prepared for him on the road. He himself did not hesitate for a moment to obey the call of duty. The Elector Frederick, who owed him at least a debt of gratitude for having helped to keep his territory free from the rapacious Tetzel, but who, both now and afterwards, conscientiously held aloof from the contest, gave proof on this occasion of his undiminished kindness and regard for him, in a letter he addressed to Staupitz. He writes as follows:--'As you have required Martin Luder to attend a Chapter at Heidelberg, it is his wish, although we grudge giving him permission to leave our university, to go there and render due obedience. And as we are indebted to your suggestion for this excellent doctor of theology, in whom we are so well pleased, ... it is our desire that you will further his safe return here, and not allow him to be delayed.' He also gave Luther cordial letters of introduction to Bishop Laurence of Wurzburg, through whose town his road passed, and to the Count Palatine Wolfgang, at Heidelberg. From both of these, though many had already declaimed against him as a heretic, he met with a most friendly and obliging reception.