Sorcery is not a religion. It has religious elements, but it is not a religion. It is a practice. From simply seeing the dictionary definition of the word "sorcery", we can conclude that it isn't necessarily religious.
But then, the question becomes, what is "religion"? I could go to the dictionary again, but this time, I think I'll just go to the Latin root words of "religion" to help cast light on the answer. "Religion" derives from the words "RE" and "LIGARE", "again" and "to bind together". Taken all together, "religion" originally meant "to bind together again" or "to bring together again".
Odd? Not at all. In the Pagan world, sacrifices for the Gods- the chief practice of Pagan religion- were done not just for the Gods, but also for the community. The people would gather around the sacrifice altar and fire and watch as some consecrated creature was killed, usually by having its throat cut, and then they would share the cooked meat, giving a portion to the God or Goddess to whom the sacrifice was devoted.
This communal meal bonded not only the worshippers with the Gods, but with one another- eating together re-affirmed bonds of friendship and community. The religious customs "brought everyone together again". And they brought people together with the Gods. Religion was the great binding force of community and the force that bound humans to the Gods.
With this in mind, we can see that any practice that brings the practitioner together with friends, families, gods, spirits, or what have you, can be described as a "religious" act. Surely when a sorcerer gives a sacrifice to whatever God or spirit he is trying to coax into helping him, he is making a religious offering. This is an unavoidable conclusion of the sheer meaning of the word.
The problem begins in the modern day, when "religion" has taken on a whole pile of other meanings. People hear the word "religion" and they immediately think of people howling and jumping around in some Christian church, or rows of Muslims bowing in mosques. They think of all the stupidity, repression, hatefulness, judgmentalism, and nonsense that is buried in the modern experience of mainstream religions.
But to allow ourselves to write off the term "religion" over knee-jerk reactions born in how people have come to think of the word in the modern day is absurd. Trying to distance ourselves from the passion-bodies in some protestant church is one thing; cutting off our own heads to save our bodies is something else. All we must do is return to the basic meaning of the word, and realize that in our personal lives, whatever "brings us together again" with one another, with our friends, our familiar spirits, or our Gods, is "religious", and it is our "religion", for lack of a better word. Some might call it their "art" or their "craft" or their "life". All linguistic preferences lead to Rome, in this case.
I discussed earlier in this essay how "religion" and "magic" were not that separate at the most ancient roots of society. I must re-state and reaffirm that idea. To reach outside of yourself, to familiar spirits or Gods or the spirits of dead ancestors or dead people, in some attempt to have them help you in some magical way, is, in a curious but direct way, religious. Sorcery requires the sorcerer to bind himself to spirits, and them to him. Sometimes, he has to do rituals to re-bind, to make the connection strong and conscious again. These are primitive and real religious acts. Intercourse with the spiritual world of any kind walks in the indeterminate country between "religion" and "mysticism" and "sorcery". These three categories aren't so different as we think.