Final answer:
According to Martin Luther, the ultimate guide for Christians was his Ninety-five Theses, which emphasized salvation by faith alone and scripture as the sole authority for Christian belief and practice.
Step-by-step explanation:
The ultimate guide to Christians according to Martin Luther is encapsulated in his Ninety-five Theses (A). Luther was concerned with the sale of indulgences and the wider implications of Church practices on the salvation of believers.
He believed that salvation could not be purchased or earned through good works; rather, it was granted solely by God's grace and could be received only through faith.
Luther's theses ignited widespread debate and were instrumental in the formation of the Protestant Reformation. His emphasis was on the authority of Scripture alone (sola scriptura) as the basis for Christian belief and practice, rather than on the traditional teachings and authority of the Catholic Church.