The primary goal of fundamental theology is to justify the credibility of Christianity. This discipline seeks the foundations of the hope Christians hold (cf. 1 Peter 3:15). One way to justify this is via ethica, which means argumentation for Christianity as the reality which both presents the highest moral values and helps to put them into practice. In his work, Joseph Ratzinger has repeatedly addressed topics related to reflection on moral values. Although these texts were not written as arguments within the via ethica, they can be an important contribution to the justification of Christianity precisely in this way. The aim of this article is to capture Ratzinger’s main theses which can inspire the development of an ethical argument in favour of Christianity. The first part of the article discusses Ratzinger’s view of contemporary trends which reject ethical values linked to the crisis of rationality and the dominant technological mentality. The second part addresses major contemporary ethical challenges, related to the functioning of democracy and individualistic understanding of the freedom of conscience. The last part presents Ratzinger’s views on how natural law and religion help discover ethical values. The conclusion summarises the main findings of the analysis. They concern, first of all, the criticism of contemporary intellectual trends which deny the importance of reflection on ethical values. Ratzinger considered such reflection necessary, basing this conviction on arguments drawn from the great philosophical tradition as well as on historical reasons. Secondly, the German theologian pointed out the most important areas in which there is a dispute over the understanding of ethical values: democracy and freedom of conscience, and showed how important in this context is the voice of Christianity together with indications which protect democracy and conscience against wrong choices. Finally, the German theologian showed how the understanding of ethical norms inherently requires reference to something that transcends human authority, namely natural law and religious traditions.