The religion known simply as "the Order" is relatively new. When Tracy Vannieu "ascended" in 378 CY, there was a small segment of the population that believed that she, not the supposed deities of the day, was to be worshipped. This gradually led to the recognition of others throughout history who had achieved great things; these people were named "Saints," and became worshipped by the new religion.
Interestingly enough, the Order gained popularity. Before, religion had been something that was rather difficult to relate to; however, with the rise of this new religion, the common folk knew the Saints. There was no mysterious greater power, no unfathomable deific might - these were once simple people who, through sheer will, had attained great things.
The upper echelons of the Order recognized this, and realized something as their clerical powers grew - the power of the divine was directly proportional to the worship given to it. Though some deities were deities simply due to sheer power, the vast majority of religious idols relied upon worship for their power.
In due time, the Order gained more followers. Those who had once followed Elendism turned to the Order, and their objects of worship became known as Eidolons.
Adhering to the theory that power increased with worship, those who were more twisted and vile than the common folk sought to bring balance to deific power; they instituted their own idols, which became known as the Lucavi.
The Order, then, consists of three seperate, though vaguely related, religions: the Order of Saints, the Council of Eidolons, and the Sect of the Lucavi.