Because of its role in debates about religion, some may mistakenly think that agnosticism itself might qualify as a religion. No one with a decent understanding of either agnosticism or religion, though, would reach such a conclusion. The easiest way to understand that is to examine the basic characteristics of religion and compare them to agnosticism: none of them apply.
Belief in Supernatural Beings
Most people probably regard belief in supernatural beings — usually, but not always, including gods — as the most fundamental and important characteristic of religions. Few religions lack belief in supernatural beings and most religions are founded upon it.
Agnosticism is the absence of knowledge of gods and, as such, does not exclude or require belief in gods; what's more, it has nothing to say about any other supernatural beings. Most important is the fact that agnosticism does not teach the existence of such beings and most agnostics in the West probably do not believe in them.
Sacred vs Profane Objects, Places, Times
Separating sacred and profane objects, places, and times is typically used by religious believers to help them focus on transcendental values and/or the existence of a supernatural realm. Agnosticism does not exclude or require believing in things that are "sacred" for the purpose of worshiping gods because it has nothing to say on the matter — neither promoting nor rejecting the distinction. Many agnostics may have things, places, or times which they consider "sacred" in that they are venerated or esteemed highly, but that's it.
Ritual Acts Focused on Sacred Objects, Places, Times
If a person believes in something sacred, they probably have associated rituals. As with the very existence of a category of "sacred" things, however, there is nothing about agnosticism which either mandates such a belief or necessarily excludes it. Rituals simply aren't relevant to agnosticism. An agnostic who holds something as "sacred" may engage in some sort of associated ritual or ceremony, but there is no such thing as an "agnostic ritual."
Moral Code With Supernatural Origins
Most religions teach some sort of moral code which is usually based upon transcendental and supernatural beliefs. Agnostics have moral codes, but they don't believe that those codes are derived from any gods — it would be a contradiction to say "I don't know if any gods exist" and "these moral values have a divine origin." It would be unusual for agnostics to believe that their morals have a supernatural origin any sort. Most important is the fact that agnosticism doesn't teach any particular moral code — agnostics all have morals, but they don't all have the same morals.
Characteristically Religious Feelings
The vaguest characteristic of religion is the experience of "religious feelings" like awe, a sense of mystery, adoration, and even guilt. Religions encourage such feelings, especially in the presence of sacred objects and places, and these feelings are usually connected to the presence of the supernatural. Agnostics may experience some of these feelings, like awe at the universe itself, but they are neither promoted nor discouraged by agnosticism itself. There is also nothing about agnosticism which teaches that such feelings have anything to do with anything sacred or divine.
Prayer and Other Forms of Communication
Belief in supernatural beings like gods doesn't amount to much if you can't communicate with them, so religions which teach that gods exist will also teach how to talk to them — typically with some form of prayer or other ritual. Agnostics don't claim to know if any gods exist so probably don't try to communicate with them — though, to be fair, that possibility isn't completely excluded. An agnostic might hope that a god exists and so try to communicate. What's most important, though, is that agnosticism doesn't teach or encourage prayer in any form.
A Worldview & Organization of One's Life Based on the Worldview
Religions are never just a collection of isolated and unrelated beliefs. Religions are entire worldviews based upon those beliefs and around which people organize their lives. Agnostics naturally have worldviews as well, but agnosticism itself isn't a worldview and doesn't promote any one worldview. Agnostics have different ideas about how to live because they have different philosophies on life. Agnosticism is not a philosophy or ideology, but it can be part of a philosophy, ideology, or worldview.
A Social Group Bound Together by the Above
A few believers follow their religion in isolated ways, but religions typically involve complex social organizations of believers who join each other for worship, rituals, prayer, etc. Agnostics belong to a variety of groups, but relatively few agnostics belong to specifically agnostic groups. When agnostics do belong to agnostic (or atheist) groups, though, those groups aren't bound together by any of the above.
Comparing and Contrasting Agnosticism & Religion
Some of these characteristics of religion are more important than others, but none is so important that it alone can cause a system to be a religion. If agnosticism lacked one or two of these characteristics, then it would be a religion. If lacked five or six, then it might qualify as metaphorically religious, in the sense of how people follow baseball religiously.
The truth is that agnosticism lacks every one of these characteristics of religion. At most, agnosticism doesn't explicitly exclude most of them, but the same can be said for almost anything. Thus, it's not reasonable to call agnosticism a religion. Agnosticism can be part of a religion, but it can't be a religion by itself. They are completely different categories: agnosticism is the absence of knowledge of some particular thing while religion is a complex web of traditions and beliefs. They aren't even remotely comparable.
So why might someone think that agnosticism is a religion? Ignorance, for one. Despite how common religion is most people don't seem to understand it very well as a general subject. There's probably even less understanding of agnosticism, even though it's a relatively simple concept. Productive discussion requires clear thinking about concepts and premises, but clear and coherent thinking are undermined by misrepresentations like this.