One may wonder what the relation is between secularism and liberalism, and why each of them is valuable. As I see it, the former can be derived from the latter, stemming from the same concerns. Liberals believe that when deciding what it is that we want the state to do, first and foremost we should prioritise the preservation of equal treatment. We do not want to create a state in which some citizens are shown greater respect than others. And the liberal argues that this has substantive implications for the state’s actions. It may not, for instance, promote a certain way of life as inherently more valuable than another. The disagreements about the good life run too deep. For the state to do this, it would have to endorse one group’s contentious claims about the best way to live over another, and there is simply no reason why citizens would want their government to do this. Why would anyone wish to set up an institution to rule over them and threaten to prosecute, for instance, those that decide they want to smoke pot? And, quite similarly, why would we agree to a state that endorses and boosts the claims of one particular religion?
According to liberals, the state should instead strive to be neutral, and merely provide a framework of security within which these decisions about how to live and what to believe are made freely by individuals. Anything less would be paternalistic, which is, by its very nature, disrespectful. It tells people what is good for them instead of leaving them to choose for themselves. And there is nothing the liberal values more than the individual’s right to exercise their own capacity for choice.
From this, various things seem to follow: not only must Church and state be formally separated so that Christianity is not embraced at the top of government as official doctrine; state tax revenue should most certainly not be used to fund schools that promote certain religions, as is the case in the UK. Even more than this, however, it seems that if the liberal is to stay loyal to his principle that choice is the goal of government, he must not only refuse to fund religious schools, but he must also ban them. This may seem the exact opposite of liberalism: a refusal to allow people to organise education freely. But the point is that if that if ‘freedom’ is being employed to diminish freedom, by presenting a certain idea of what goodness is to children as fact, the idea that education should push no agenda and serve only to develop certain faculties which allows kids to decide for themselves is inevitably attacked, and the liberal thus has a very good reason to prevent any measures that are likely to undermine a child’s future choices.
So the answer is that secularism and liberalism are intrinsically connected, but secularism is narrower, perhaps best seen as a subset of liberalism. It addresses questions about the role of religion and what the state should do about it. Liberalism deals with these questions more broadly, and says, for instance, not only should the state not promote Christianity, but it should avoid criminalising any activity which is not directly harmful to other people. So not only is the Church ousted and drug laws repealed, but public masturbation gets the green light, and so does incest. If people decide this is the way they wish to live, what right do we have to intervene and tell them that we know better?