This article elaborates the term Logos in two fictitious letters of Candidus, which Marius Victorinus wrote to present Arian points of view concerning the Trinitarian debate in the middle of the 4th century. The article investigates these two short letters and their historical and theological sources to demonstrate Marius Victorinus’ knowledge and understanding of the Arian controversy and the mystery of the Triune God. Although he wrote these letters himself, this research seems to be a particularly important in the interpretation of Marius Victorinus’ theological views and arguments presented in his writings against the Arians, in which he undertakes the most difficult questions concerning the unbegotten and simultaneously begetting God.