This article intends to study the Christological interpolations that can be found in the text of the Greek formula quarta of the pseudo-Athanasian Symbolum Quicumque. In particular, this study has a threefold objective: to show that the so-called textus receptus – as it is printed in Usserius and Migne – is not the primitive text of the formula quarta; to analyze the four Christological interpolations and their probable sources; and to hypothesize the origin of these insertions. In this sense, the reworkings of the primitive text of the formula quarta quote or allude to specific Patristic and Byzantine sources, such as the Expositio rectae fidei by Theodoret of Cyrus (pseudo-Justin), a pseudo-epigraph letter to Peter the Fuller, the Expositio fidei (De fide orthodoxa) by John of Damascus, and the Ad monachos de fide by Nicephorus Blemmydes. Furthermore, the interpolations can be placed between the second half of the 13th and the first decades of the 14th century and might have been inserted by one of the scholars who served under the emperor Andronicus II Palaeologus – perhaps by Joseph Rhakendytès the Philosopher – or by some other anonymous Byzantine compiler. At the same time, the presence of the interpolated texts in the earliest complete witnesses of the horologion Thekaras opens another interesting path for researching the author of these insertions in the formula quarta.