A few thoughts
The hallmark of a great religion is that it is open, evolves continuously, enriches itself with new ideas and allows its followers to explore and understand other religions too.
This is a great standard, and one that is often asserted, but by definition I can't think of any "great religions". Some of Sciforums' anti-Catholics have in the past ridiculed the RCC church as selling out to idolaters and pagans for "enriching itself with new ideas and allowing its followers to explore and understand other ideas."
To the other, Sufism, while it demands an early association by the student to the principles of Islam, sheds all religious observance later in training; furthermore, Sufis assert to exist in many religions despite their Islamic centerpiece.
I recall that A.L. Basham, in
Classical Hinduism, noted that in the history of Hindu, back even to the Aryans, there has existed a condition which was coined by famed sociologist Max Muller called "henotheism", in which major paradigm shifts are easily assimilated. In other words, whatever is the prevailing God's name is the prevailing God; it's all the same thing so it doesn't matter. This is partially because the major paradigm shifts were not particularly major; rather, the stylizing of the deities that came with the passage of time.
And there are a number of witchcrafts that have a single rule:
An thou harm none, do what thou wilt. While these religions allow their followers to absorb whatever material the conscience finds suitable, one can hardly call any of them "great religions" because they aspire to loosely-based memberships numbering in the thousands, not in the millions or billions.
Two cents to go on, hopefully.
thanx,
Tiassa