What does IUS stand for? 

Ius

Ius or jus is Latin for one sense of the English word, law. In the Canon Law of the Catholic Church, ius refers to custom, practice or "Tradition." The early law of the Church, especially prior to the First Council of Nicaea in 325 a.d., was largely unwritten, at least in the form of law, but existed in the practices, customs and teachings of the early Christian community. What largely was communicated generation to generation was an oral tradition passed from the apostles to the Bishops, and from Bishops and priests to the faithful through their preaching and way of life. Some of what is included in the term ius would be interpretations of particular scriptural passages, theological understandings of the liturgy and liturgical practices themselves. Evidence for the content of this oral tradition of teaching is found among the writings of the Early Church Fathers as well as in the later legislation of the Church or lex. Ius is typically understood in contradistinction to lex. The Early Church, which existed more or less under persecution in the Roman Empire prior to Constantine I in the early fourth century, was not in a position to gather large councils for the purpose of legislation or theological clarification prior to 325 a. d. Laws formalized as lex after 325 a.d. are sometimes falsely interpreted as having a "new" content. This is usually not the case. Most Church legislation is either a development of prior teaching, or practice or re-affirmation of teaching or practice unless otherwise expressly stated.

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