«Liber de fide» by Rufinus the Syrian is a source whose significance in the history of the Pelagian controversy is assessed in different ways. The aim of the publication is to establish the place of this work in the theological and literary context of the Pelagian controversy of the early fifth century in order to determine the specifics of the polemics at an early stage. To achieve this goal, various hypotheses concerning the authorship of the «Liber de fide» as well as the person of Rufinus himself are examined. The testimony of St. Augustine, which cites the words of Celestius at the ecclesiastical court of 411, in which he mentions the presbyter Rufinus denying the transmission of sin; the testimony of Marius Mercator and St. Jerome are examined. On the basis of these sources, the hypothesis of attribution of the text to Rufinus of Aquileia is denied. The theological connection of the «Liber de fide» with the Antiochian tradition and, in particular, its similarity to the exegesis of Theodore of Mopsuestia allow us to suggest that Rufinus the Syrian was familiar with his writings and brought Antiochian ideas to Rome. In this connection, the publication considers various hypotheses concerning the dating of the «Liber de fide», resulting in the date of the text being determined as the end of the 4th – beginning of the 5th centuries. A chain of influence is proposed: Theodore of Mopsuestia (late 4th – early 5th centuries) — Rufinus the Syrian — Celestius — Pelagius — Julian of Eclanum — Theodore of Mopsuestia. The second part of the publication offers a translation of the «Liber de fide», with a textual and real commentary, with emphasis on intertextual parallels with the writings of Theodore and Pelagius as well as with the anti-Pelagian polemical works of St. Augustine, especially with the treatise «De peccatorum meritis et remissione et de baptism parvulorum», the statements of opponents criticised in which find close parallels in the text of the «Liber de fide». The commentary shows that the text of the «Liber de fide» contains an original theological exposition of Theodore's ideas, but that it differs from the work cited by St. Augustine in 411. It is argued that the way the polemical argumentation of the «Liber de fide», written in the context of theological disputes of the late 4th and early 5th centuries, determines the vector of polemical strategies and the stylistics of argumentation in the anti-Pelagian writings of St. Augustine, who may have had before him the «Liber de fide» as revised by Celestius.