linguistic:paraphrasing
Trinity

Although the Western churches do not consider the Eastern and Western understanding of the Trinity to be radically different, Eastern theologians such as John Romanides and Michael Pomazansky argue that the Filioque clause is symptomatic of a fatal flaw in the Western understanding, which they attribute to the influence of Augustine and, by extension, to that of Thomas Aquinas.

Filioque, Latin for "and (from) the Son", was added in Western Christianity to the Latin text of the Nicene-Constantinopolitan Creed, which also varies from the original Greek text in having the additional phrase Deum de Deo (God from God) and in using the singular "I believe" (Latin, Credo, Greek Πιστεύω) instead of the original "We believe" (Greek Πιστεύομεν), which Oriental Orthodoxy preserves. The Assyrian Church of the East, which is in communion neither with the Eastern Orthodox Church nor with Oriental Orthodoxy, uses "We believe".

Filioque states that the Holy Spirit proceeds from the Son as well as from the Father, a doctrine accepted by the Catholic Church, by Anglicanism and by Protestant churches in general. Christians of these groups generally include it when reciting the Nicene Creed. Nonetheless, these groups recognize that Filioque is not part of the original text established at the First Council of Constantinople in 381, and they do not demand that others too should use it when saying the Creed. The Catholic Church does not add the phrase corresponding to Filioque (καὶ τοῦ Υἱοῦ) to the Greek text of the Creed, including in the Roman Rite of Latin Church Catholics.

At the 879–880 Council of Constantinople the Eastern Orthodox Church anathematized the Filioque phrase, "as a novelty and augmentation of the Creed", and in their 1848 encyclical the Eastern Patriarchs spoke of it as a heresy. It was qualified as such by some of the Eastern Orthodox Church's saints, including Photios I of Constantinople, Mark of Ephesus, and Gregory Palamas, who have been called the Three Pillars of Orthodoxy. The Eastern church believes by the Western church inserting the Filioque unilaterally (without consulting or holding council with the East) into the Creed, that the Western Church broke communion with the East.

Eastern Orthodox theologians such as Vladimir Lossky criticize the focus of Western theology of God in 'God in uncreated essence' as misguided, which he alleges is a modalistic and therefore a speculative expression of God that is indicative of the Sabellian heresy. Eastern Orthodox theologian Michael Pomazansky argues that, in order for the Holy Spirit to proceed from the Father and the Son in the Creed, there would have to be two sources in the deity (double procession), whereas in the one God there can only be one source of divinity, which is the Father hypostasis of the Trinity, not God's essence per se. In contrast, Bishop Kallistos Ware suggests that the problem is more in the area of semantics than of basic doctrinal differences:

The Filioque controversy which has separated us for so many centuries is more than a mere technicality, but it is not insoluble. Qualifying the firm position taken when I wrote The Orthodox Church twenty years ago, I now believe, after further study, that the problem is more in the area of semantics than in any basic doctrinal differences.