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In its ecclesiastical setting, Nicolaitans means the bishops and prelates of the Church have gained a triumphal victory or conquest over the laity. Members are compelled and forced to submit to the arbitrary dominion of men who have become that thing which the Eternal hates. The Apostle Peter warned that those who were leaders among God's people were NOT to dominate over the faith of others but rather exhort them to do right (1Peter 5:1 - 3).
The teachings and preaching of this group is also variously defined as a hierarchy, the power of dominion, or as the establishment of a government by ecclesiastical rulers.
The teachings of the Nicolaitans reared its head in one of the decisions arrived at during the Catholic Church's Council of Trent, held between 1545 and 1563. They stated, "If anyone shall say that there is not in the Catholic Church a hierarchy established by the divine ordination, consisting of bishops, presbyters and ministers, let him be anathema." The entire top-down structure of the Catholics, as well as many other churches and denominations, owes its survival to maintaining what the Nicolaitans taught.
The doctrine of the Nicolaitans is the teaching that there is a strict hierarchy within God's church that must be respected by all. Ranks and levels are created in order to maintain power and control so that, ultimately, the lowest level of the church (the members) can be ruled over and taken advantage of at any time. The whole system feeds on competition and strife among those who consider themselves Christians.
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http://www.biblestudy.org/basicart/why-does-god-hate-practices-of-the-nicolaitans.html