A "creed" is a statement of belief. Usually this term is used for a formal statement of the religious beliefs of a church, like the classic "Apostles' Creed": "I believe in God the father almighty, maker of Heaven and Earth, and in Jesus Christ his only son our lord, who was conceived by the virgin Mary", etc.
People sometimes also talk of the creed of a political group or a school of thought in some academic discipline, like "the creed of Keynesian economics". But that isn't relevant here.
By extension, a "creed" can be a group of people who hold to a certain set of beliefs. That is the sense in which the writer is using "creed" here.
The other issue here is the phrase "favor ... over". We say that someone favors group or thing A over B when he likes A better than he likes B, or when he gives some special privilege or honor to A that is not given to B.
So the writer here is contrasting the ROC -- I assume that stands for Russian Orthodox Church -- with other "creeds", that is, other churches. He is saying that the OCSE does not give any special honor or privilege to the Russian Orthodox Church that it does not give to every other church.