The document said clarification was necessary because an ambiguous use of the phrase, putting the Catholic Church on equal footing with other churches, had become ``prevalent in contemporary writings on ecumenism.''
The four-page document, accompanied by a letter from Cardinal Joseph Ratzinger, prefect of the Congregation for the Doctrine of the Faith, was signed June 30 and distributed to the heads of bishops' conferences.
Catholic News Service obtained a copy of the letter and the document Aug. 23.
The document said the expression ``sister churches'' could only be properly used as a way of describing the relationship between local Catholic churches, like ``the church of Rome,'' and non-Catholic churches.
But the Catholic Church as such could not be described as a ``sister church,'' it said.
``The one, holy, catholic and apostolic universal church is not sister but `mother' of all the particular churches,'' the document said.
``This is not merely a question of terminology, but above all of respecting a basic truth of the Catholic faith: that of the unicity of the church of Jesus Christ. In fact, there is but a single church, and therefore the plural term churches can only refer to particular churches,'' it said.
``Consequently, one should avoid, as a source of misunderstanding and theological confusion, the use of formulations such as `our two churches''' because it might seem to imply that there is more than one church of Christ, it said.
The document also said that Catholic use of the phrase ``sister churches'' could only apply to ``those ecclesial communities that have preserved a valid episcopate and Eucharist,'' like the Orthodox churches.
In his letter, Cardinal Ratzinger said the document was approved June 9 by Pope John Paul II and therefore was ``to be held as authoritative and binding.'' But the document will not be published in the Acta Apostolicae Sedis, the official periodical of Vatican legislation, because of its ``limited purpose of specifying the correct theological terminology on this subject,'' he said.
Cardinal Edward I. Cassidy, president of the Pontifical Council for Christian Unity, declined Aug. 24 to comment on the document.