He was now not only a pastor, but also a teacher. At the beginning of the year 1539, Capito, struck with his gifts for theological teaching, entreated him to join that office with his pastorate. Although he felt reluctant to do so, from his sense of the difficulty of that ministry, he at length consented. Every day he preached in the church of St. Nicholas, in which he taught the students of the academy. The interpretation of the Scriptures was for him the basis of theological science, and for his exposition he selected two of the richest books of the New Testament--the Gospel of St. John and the Epistle to the Romans. His plan was to search out the meaning of the sacred writer, and to set it forth with an easy ‘brevity which did not entail obscurity;’ and for that purpose ‘he took pains to regulate and proportion his style.[752]’ In his view the Epistle to the Romans was ‘a path to the understanding of the whole Scripture.’ Some doctors attended these lectures, and expressed their high admiration.[753] He did not content himself with being at the same time pastor and professor, he also worked diligently in his study. He revised his _Institution_, and prepared a second edition; he recast his _Catechism_; he composed a treatise on the Supper, of which he sent a copy to Luther. Calvin, like Zwingli, regarded the bread and the wine as signs, as pledges that Christ gives to the believer his crucified body and his shed blood; that is to say, communicates to him the expiatory virtue of his death. He taught that the believer receives the body and the blood by faith, which is _the mouth of the soul_, and not by the bodily mouth. But he differed from the reformer of Zurich in that he saw in the supper a mysterious union with the glorified person of Christ. ‘With good reason,’ he said, ‘the bread is called body, since it not only _represents_ him, but also _presents_ him to us. We must therefore really receive in the supper the body and the blood of Jesus Christ, since the Lord sets forth to us therein the communion of both. If God gave us only bread and wine, leaving behind the spiritual truth, would it not be the case that he had instituted this mystery on fictitious grounds?[754] This alliance is effected on our part by faith, and on the part of God by his secret and miraculous virtue. The Spirit of God is the bond of this participation; that is why it is called spiritual. When Luther began his course, he appeared to say that the bread was the body of Christ. Œcolampadius and Zwingli appeared to leave in the supper nothing but the bare signs without their spiritual substance. Thus Luther failed on his side, Zwingli and Œcolampadius on their side. Nevertheless, let us not forget the grace which the Lord gave to all of them, and the benefits which he has conferred on us by their instrumentality.’[755]