To be secular means, simply, to be without religion. But this adjective applies very differently to countries than it does to people. For a person to be secular means that they do not subscribe to the creed of any existing religion (as in "secular humanist"). A secular person, by the usual and conventional meaning of the term, is therefore a nonbeliever, an atheist.
For a state to be secular has a superficially similar meaning: that that state is not ruled by a religious authority, nor are its laws derived from the tenets of a religious creed, nor are its citizens required to give assent to a particular set of religious beliefs. A secular state is one that has no official religion, no favoritism or preference shown by the government for one church over another, or for religious belief in general as opposed to nonbelief.
But this does not mean that a secular state must be one that officially bans the holding of religious beliefs among its citizenry or takes positive steps to disadvantage religion. Just because a state is secular says nothing about the religious affiliations of the people who make up that state. The people themselves may be devout followers of any number of faiths. Even the elected leaders can hold whatever beliefs they like, so long as they make laws based on religiously neutral considerations of public policy, and not on the edicts of any particular sect.
The difference is a crucial one, and partisans of the religious right have effectively stirred up confusion on this point in order to argue that because a secular person has no religion, a secular state must also contain no religion whatsoever. But this is not true. Because people are unified wholes, a secular person has no religious beliefs and that's that. But states and countries are not single entities, but assemblages of large numbers of people who may believe all sorts of different things. The key is that the different beliefs of all these people "average out", so to speak, producing a government that treats them all as equals before the law, regardless of their religious beliefs or lack thereof.
The Army veteran and atheist Ron Garrett put this point succinctly in an essay on Ebon Musings:
Who gets to decide we have to believe in one god, or many gods, or no gods? Under our Constitution, only I get to decide that for me, and you for you, and neither of us gets to use our temporary control of the government to bully others into agreement, or program children in a public school classroom to believe in someone else's gods and prophets against the parent's wishes, or even in accordance with them, because the government can't do that under our law.
The problem arises when the government does not treat all people as equals, but begins to show special favoritism or grant official privileges to certain religious beliefs and not others. This is when advocates of secularism - who may, or may not, be secular people themselves - step in to demand that the government cease this favoritism and return to its neutral position. And this, inevitably, is when the religious right partisans appear to create confusion. They argue that what we secularists (secretly) want is not neutrality of government towards religion, but official support for atheism and positive hostility of government towards religion.
Phrased in this way, the argument is absurd. First of all, as I previously mentioned, many people who support secularism are not secular people themselves: they are religious believers who recognize the value of government remaining neutral toward religious beliefs. (For example, Barry Lynn, the current director of Americans United for Separation of Church and State, is an ordained minister in the United Church of Christ.) Second, if that was what secular people wanted, we wouldn't merely be demanding that the government cease its official endorsement of religions we don't belong to; we would be demanding that the government take our side. We would be demanding not that "under God" be stricken from the Pledge of Allegiance, but that it be replaced with "under no God". We wouldn't be asking for an end to official prayer before city council meetings; we'd be asking for an official reading from the books of Richard Dawkins or Robert Ingersoll.
The rampant dissemination of falsehoods about secularism has led to absurd situations where religious fundamentalists and secularists have exactly the same goal, only the former group doesn't realize it. For instance, there have been many cases where a state or township put some intrusive religious monument on public property like a school or a courthouse, was sued by a church-state separation group, and after a protracted and bitter legal fight, finally agreed to move the monument to private land, like a church lawn, in exchange for the dropping of the complaint. Inevitably, religious fundamentalists cheer this outcome as if it represented a victory over those evil secularists, when in fact it's the very thing we were asking for all along.
Private individuals and religious groups can promote their religion to their heart's content, on their own property and with their own money. But the government is supposed to represent all of us, to respect everyone's freedom to believe or not believe as they see fit, and to refrain from any action which sends the message to a particular religious (or atheist) group that they are favored, privileged insiders, or that they are disfavored, rejected outsiders.
And lastly, why do we advocate secularism in government? The answer is that, in the long run, it's better for everyone regardless of what beliefs they hold. Prominent advocates of state secularism, like America's founding fathers, had the lessons of history to draw from, and knew full well that when a state is not secular, when its religious belief is determined by its rulers and competing beliefs are outlawed or persecuted, the inevitable result is bitter, bloody religious war. Europe was convulsed by warfare between Catholics and Protestants for centuries for that very reason. Today we see similar strife between Sunni and Shi'ite Muslims, in countries where the rulers demand allegiance to one or the other of those sects.
One doesn't have to be an atheist to recognize that it's better for everyone when the state doesn't attempt to coerce uniformity among its citizens. Religious belief is and must be a matter of individual conscience, not of coercion imposed by law. That's why secularism in government is good for us all, both for religious people and also for people who are secular themselves.
But I would add that, for individuals, secularism is a good thing as well. Being a secular person means being free from the superstitious and arbitrary impositions of religion, from the chains of senseless dogma and from archaic and irrational rules. Secularism means the freedom to live your life as you see fit, pursuing whatever purpose you choose for yourself, subject only to the limitation that you allow others the same freedom and not interfere with their equal right to set their own course. As Robert Ingersoll put it in his usual poetic style:
Secularism believes in building a home here, in this world. It trusts to individual effort, to energy, to intelligence, to observation and experience rather than to the unknown and the supernatural. It desires to be happy on this side of the grave.
Secularism means food and fireside, roof and raiment, reasonable work and reasonable leisure, the cultivation of the tastes, the acquisition of knowledge, the enjoyment of the arts, and it promises for the human race comfort, independence, intelligence, and above all liberty.
Secularism, in government and in individual life, is something the human race needs more of. We should work to make it so.
(courtesy of Daylight Atheism)