It is impossible to exaggerate the services of the men who, deeply impressed with the gravity of the crisis, threw themselves into the breach, with exhortations to their co-religionists to remain faithful to their creed. In defiance of the dangers which menaced them, they scattered their inspiriting discourses far and wide. Foremost among them were the men who had distinguished themselves at the Tortosa disputation by their unyielding attitude and their courage in withstanding the unjustifiable attacks upon the Talmud-- Don Vidal (Ferrer) Ibn-Labi and Joseph Albo. The former drew up in Hebrew a refutation of Geronimo's impeachment of the Talmud (Kodesh ha-Kodashim), and the latter circulated, in Spanish, an account of a religious controversy he had sustained with an eminent church dignitary. Isaac ben Kalonymos, of a learned Provençal family named Nathan, who associated a great deal with learned Christians, and frequently had to defend his religious convictions, wrote two polemical works, one entitled "Correction of the False Teacher," directed against Geronimo's libelous essay, and the other, called "The Fortress," of unknown purpose. He also compiled a laborious work of reference intended to assist others in defending Judaism from attack. Isaac Nathan, in his intercourse with Christians, often had to listen to criticisms of Judaism, or evidences drawn from the Hebrew Bible, in favor of Christian dogmas, which he found were always based on false renderings of Hebrew words. To put an end to these illusory outgrowths of prevailing ignorance of the original text of the Scriptures, or, at least, to lighten the labors of his brethren in refuting them, he resolved to compile a comprehensive digest of the linguistic materials of the Bible, by which the actual meaning of each word should be made clear. According to the plan adopted, any one can ascertain, at a glance, both how often a certain word occurs in the Bible, and its varying meanings according to the contexts. The work thus undertaken by Isaac Nathan was of colossal scope, and occupied a long series of years (September, 1437-1445). It was a Bible concordance, that is, the verses were grouped alphabetically under the reference words according to roots and derivations. The existing Latin concordances served in a measure as models, although their purpose was the less ambitious one of assisting preachers to find texts. Isaac Nathan, who produced various other works, by this concordance rendered inestimable and lasting service to the study of the Bible, although his labor was of a purely mechanical kind. Originating from the temporary needs of the polemical situation, it has been, and will ever remain, a powerful weapon for ensuring the triumph of Judaism in its struggles with other religious systems.