In Christology, the Logos (Greek: Λόγος, lit. ''Word", "Discourse", or "Reason'') is a name or title of Jesus Christ, seen as the pre-existent second person of the Trinity. It has been important for establishing the doctrine of the divinity of Jesus and his being God the Son by Trinitarian theologians as set forth in the Chalcedonian Creed. This was the first Council not to be recognized by any Oriental Orthodox church and was a primary cause for meeting during the First Council of Nicae and eventual separation. The Chalcedonian Definition was written amid controversy between the western and eastern churches over the meaning of the Incarnation (see Christology), the ecclesiastical influence of the emperor, and the supremacy of the Bishop of Rome.
The concept derives from the prologue to the Gospel of John, which is often translated into English as: "In the beginning was the Word, and "the Word was with God" verses "the Word was God." In the translations, "Word" is used for Λόγος, although the term is often used transliterated but untranslated in theological discourse. The Oriental Orthodox and Assyrian churches use this profession of faith with the verbs in the original plural ("we believe") form, but the Eastern Orthodox and Roman Catholic churches convert those verbs to the singular ("I believe").