Is the Golden Dawn a religion?

This is a common query from those new to the Golden Dawn, and the answer is: no, the Golden Dawn is a system of ceremonial practice. It is a teaching and initiating order, designed to promulgate spiritual and occult knowledge in a steady, structured manner. You can be a member of any religion and still be a member of the Golden Dawn or utilise any of its teachings or practices. To best illustrate this, here is a quote from the Hierophant in the Neophyte initiation before the Oath is taken:

"There is nothing contrary to your civil, moral or religious duties in this Obligation."

The above clearly states that there is nothing in the obligation or teachings of the Order that will run contrary to your religious duties (i.e. you will not be asked to pray to or worship any God that is not your own). While the teachings may contradict the teachings of exoteric religion, your inner relationship with the Divine is seen as a personal thing and is never interfered with. Thus the generic term "Lord of the Universe" is often used in reference to the Divine. This term, while masculine and of Gnostic origin, can be taken to represent whatever form of Divinity you personally prescribe to.

There are also the words of the Hiereus in the exhortation towards the end of the 0=0:

"Remember that you hold all Religions in reverence, for there is none but contains a Ray from the Ineffable Light that you are seeking."

This respect and reverence for all religions is also indicative of the fact that the Golden Dawn does not require its members to be of a certain religion, but to merely be tolerant of all religions. This is exemplified in the synthetic nature of the Order itself, drawing here and there from multiple traditions: Christian, Judaic, Pagan, Gnostic, Platonic, Rosicrucian, and more.

It is important to note, however, that the Golden Dawn, given its synthetic nature, utilises terminology from, for example, the Qabalah, which is Jewish at heart. Thus, if you are uncomfortable with Judeo-Christian symbolism, you may wish to read the ceremonies and teachings of the Order to ensure you will feel right about utilising such symbolism and terminology. The Outer Order is primarily Egyptian in symbolism (and thus Pagan), while the Inner Order is primarily Rosicrucian and Gnostic in symbolism (and thus Christian). Members of the Order have in the past and present been of multiple religions, though primarily Christian, Jewish, and Pagan.

Relating to this topic is that of Pagans & the Golden Dawn.

Out of interest, I have started a poll on the new Golden Dawn Forum as a kind of census of the religions upheld by people working with the Golden Dawn system. You can find this poll here.