Fundamental Theology is scientific discipline which ex professo treats about justifying the credibility of Christianity. This discipline is about the foundations of faith and justifies credibility of Divine Revelation which has been fulfilled in Jesus Christ and lasts in the Church. Fundamental Theology would like to make Christian faith reasonable as it shoud be because a man is reasonable and free person full of dignity. Contemporaty Fundamental Theology not only makes the traditional argumentation reacher but also tries to find new arguments for the credibility of Christianity. This article contents following issues: 1. justifying the credibility of Christianity as the aim of Fundamental Theology; 2. some projects rejecting the necessity of justifying the credibility of Christianity; 3. traditional arguments for the credibility of the Revelation fulfilled in Jesus Christ; 4. justifying in modern Fundamental Theology. The process of justifying in Fundamental Theology means searching or making arguments, reasons, for supernatural character of Christianity. Because every argument shows the Revelation and faith only partially, so each of them cannot fulfill the whole task. Fundamental Theology needs many arguments. Presenting lots of them Fundamental Theology is able to justify Christian faith more fully.