These two treatises may be regarded as the immediate occasion for the writing of the _Babylonian Captivity_, which is, however, in no sense a direct reply to either of them. "I will not reply to Alveld," Luther writes on August 5 to Spalatin, "but he will be the occasion of my publishing something by which the vipers will be more irritated than ever." (Enders, II, no. 335; Smith, I, no. 283.) Indeed, he had promised some such work more than half a year before, in a letter to Spalatin of December 18, 1519: "There is no reason why you or any one else should expect from me a treatise on the other sacraments [besides baptism, the Lord's supper, and penance] until I am taught by what text I can prove that they are sacraments. I regard none of the others as a sacrament, for there is no sacrament save where there is a direct divine promise, exercising our faith. We can have no intercourse with God except by the word of Him promising, and by the faith of man receiving the promise. _At another time you shall hear more about their fables of the seven sacraments._" (Enders, II, no. 254; Smith, I, no. 206.) Thus the _Prelude_ grows under his hand and assumes the form of an elaborate examination of the whole sacramental system of the Church. He makes short work of his two opponents, and after a few pages of delicious irony, of which Erasmus was suspected in some quarters of being the author, he turns his back on them and addresses himself to a positive and constructive treatment of his larger theme, lenient toward all non-essentials, but inexorable with respect to everything truly essential, that is, scriptural. The _Captivity_ thus represents the culmination of Luther's reformatory thinking on the theological side, as the _Nobility_ does on the national, and the _Liberty_ on the religious side. It sums up and carries forward all of his previous writings on the sacraments, just as, nine years later, the _Catechisms_ gathered up and moulded into classic form his writings on catechetical subjects. Passage after passage, often whole pages, from the _Resolutiones disp._, the _Treatise on Baptism_, the _Conitendi Ratio_, the _Treatise on the New Testament_, the _Treatise on the Blessed Sacrament_, are transferred bodily to this new and definitive work, and find in it the goal toward which they had been consciously or unconsciously tending. The reader is referred to a fine comparative study in Köstlin's _Theology of Luther_ (English trans.), I, 388-409. The title is a reminiscence from the _Resolutiones super prop, xiii._, of 1519,--"absit ista plus quam babylonica captivitas!" The sense in which the work is called a "prelude" is explained on page 176; the theologian in Luther could not deny the musician, he goes into battle singing and comes back with the stanza of a hymn upon his lips.