Liberalism is the ideology that is primarily concerned with liberty, above all else. Putting liberty first is the defining feature of liberalism, therefore. However, this cannot be liberalism’s only feature, for liberty is also found in various forms in other ideologies. For example, in traditional feudal societies with absolute monarchies, the King had almost unlimited liberty. The lords also had an amount of liberty much greater than any citizen in a modern liberal democracy: for example, they had the ‘liberty’ to own and trade slaves. The unique thing about liberalism is that it aims to distribute as equally as possible the liberty of each person in society. Therefore, while nobody can have the liberties of kings and nobles past, everyone can have their fair share of liberty. While liberals disagree on how liberty can be distributed most equally, with some arguing for NAP-based libertarianism and others arguing for a strong welfare state, this often unspoken shared principle is what we have in common.
How does liberalism’s dedication to distributing liberty equally make it a moral ideology? To answer this question, we need to first look at what liberty is. Liberty is the power an individual has over their own actions, their ability to put their ideas into action. Therefore, looking at it from a moral perspective, liberty is moral agency, i.e. the ability to act in accordance with one’s moral compass. An equitable distribution of liberty therefore ensures an equitable distribution of moral agency. In this way, liberalism ensures that every individual in society has an equal share of moral agency. At this point, we need to turn to the fact that liberty (and hence moral agency) are also finite resources: if some have more, others must have less. If lords can command slaves (therefore having more liberty), slaves will not be able to act according to their own moral compass, and thus have no moral agency. Therefore, in an equal distribution of liberty (and hence moral agency), everyone can have full moral agency over their own beliefs and actions, but nobody can have moral agency over another. This, I would argue, makes liberalism the ONLY morally valid ideology.
I believe that the principle of equality of moral agency is the most important principle in political morality, because it is the only way of distributing liberty (which is one and the same as moral agency, liberty being from a political rights perspective and moral agency from a moralistic perspective) that is consistent with the fact that every human being has equal moral standing from birth, and the fact that all human beings are flawed in some ways (i.e. not perfect and not capable of knowing the absolute truth in every sense). Let’s start with the equal moral standing part. In fact, the founding fathers of the United States were some of the first people to recognise this: in their words, everyone was created equal, and therefore everyone has certain inalienable rights. Inalienable is the keyword here: these rights cannot be rightfully taken away by any action of any external authority, whether the authority comes from hereditary privilege or from collective mandate. Any action that compromises these inalienable rights are morally illegitimate. To believe that every human being is born equal morally requires that liberty (and hence moral agency) is distributed equally; in other words, a failure to distribute liberty equally is morally inexcusable, unless you happen to believe that people are not born equal. Now, let’s look at the fact that all human beings are flawed. Because all human beings are flawed, nobody can certainly make the right decisions every single time. To allow one human being to make moral decisions for another against their will therefore potentially means forcing someone to commit a moral wrong even while knowing it is wrong. Furthermore, allowing those in power to make moral decisions for everyone else means that, when the leader of a country doesn’t get it right, the whole country commits a moral wrong. The worst example of this in all human history would be the Holocaust
For more about Moral Liberalism, read TaraElla's book The Moral Libertarian Horizon.