A definition of Humanism from the American Humanist Association's website:
Humanism is a progressive philosophy of life that, without theism and other supernatural beliefs, affirms our ability and responsibility to lead ethical lives of personal fulfillment that aspire to the greater good of humanity.
For the moment, let's take that definition of Humanism at face value and also, let's assume that the term "progressive" as used in the AHA's definition is roughly analogous to the way "progressive" is commonly understood as it refers to a political or economic position.
Progressivism should not be confused with liberalism, though there is some overlap in views. Both progressives and liberals are concerned with the protection and expansion of basic human rights, which manifests itself in the current political climate as battles over reproductive rights, women's rights, LGBTQ equality and many other issues. Progressives part company with liberals on issues of economic fairness. Though it would be impossible to do justice to progressive ideas in so short an essay, in general progressives want corporations and the so-called 1% to pay their fair share in taxes, and they want corporations to be prevented from price gouging, polluting the environment or committing other forms of malfeasance via wise government regulation.
This is why Obama can be a liberal, in that he favors marriage equality and Medicare, but not be a progressive, in that he won't do much to regulate or curtail the irresponsible behavior of the large financial institutions responsible for our current economic crisis.
Progressivism is more of an economic than political stance, but it is impossible to fully separate the political from the economic. The overlap between Humanism and Progressivism begins with the shared core values of each. Progressives, like Humanists, value Compassion. They value individual human beings, and work to better their lives. This compassion manifests itself in social justice work, but it also comes through in policies that favor the rights of workers over the "rights" of corporations, which are non-human legal constructs.
Progressives, like Humanists, believe that humans can make the world a better place, and though many Progressives may believe in God or some higher power, they tend to believe that the betterment of our lives and society comes through the application of human ingenuity and hard work, not by divine fiat. Humanists and Progressives embrace Optimism about the future not as some irrational fantasy but as a carefully considered and reasonable position to hold. If it is possible to improve the lives of humans in this world, then we should certainly try to do so but if it is in fact beyond the ability of humankind to improve the world, we will only learn this for sure by trying. Either way, it makes sense to try.
Though there is some overlap between Humanist and Progressive membership there is a large contingent of people who identify as one or the other, but not both. Though it would be impossible to outline every reason why a person might embrace one label and reject the other, some general observations can be made.
-Not all Progressives are comfortable identifying themselves as non-theists and/or rejecting a belief in the supernatural. Many Progressives derive comfort from religious belief.
-Not all Humanists consider Progressive economics to be a reasonable position to hold given their interpretation of the evidence. Many prefer economic theories that tend towards laissez-faire capitalism, Libertarian or even Objectivism.
As these two exceptions illustrate, the key value in dispute here is Reason. Humanists, utilizing Reason, have done away with the need for God and, in embracing the scientific method, have rejected the existence of other supernatural beings, such as ghosts and demons, pending evidence to the contrary. Many Progressives have not made that cognitive leap, and continue to find value in believing in the existence of God.
However, if a Progressive believes that it is only through human effort that the world can be made a better place, if in other words, a Progressive feels that the God he or she believes in is remote from the vagaries of our day-to-day existence, then what becomes the functional difference between the points of view? In his most recent book Beyond Religion, the Dalai Lama argues for the existence of a secular ethics, divorced from God, Karma or the supernatural. Note that he does not explicitly reject the existence of the supernatural, he simply deigns the question irrelevant to the question of creating a universal human ethos. He has embraced some of the core values of Humanism to reach this insight, especially Reason and Compassion.
What the Dalai Lama did in his book is not all that exceptional. I believe that most Progressives have reached similar conclusion about their faiths, if they have one. I think the majority of people who self-identify as Progressives and believe in God believe in the kind of God who at his most interventionist can be described in the motto, "God helps those who help themselves."
Many Humanists with a strong atheist or anti-theist streak will object to the idea of labeling those with some form of "weak theism" Humanists, and I might concur and endorse a term like proto-humanism or quasi-humanism to describe such positions, but I would reject the idea of calling such people religious humanists because, as I argue, religion is not all that important to their view of secular ethics and action, and the term "religious" is so loaded it overpowers the term "humanism" when used in conjunction.
From the point of view of a Humanist and an atheist interested in advancing a secular, Humanist and Progressive society, there is some compelling scientific research that seems to indicate that progressive economic policies and a strong social safety net actually decrease the need for belief in God and the supernatural. The secularization thesis, described in this AlterNet post by Amanda Marcotte, holds that as societies become more egalitarian they become less religious. Marcotte's closing provides a possible sign post:
Religion’s grip on power is tightly entwined with the economic misfortunes of the people. If we want to build a more secular society, the first step is building a more equitable one.
Those Humanists who persist in advancing forms of Capitalism dependent on political policies that seek to do away with government oversight, regulation and progressive tax policies are embracing economic theories that cut against the value of Reason and their own best interests. To the extent that these Humanists reject reason when it comes to wise economic and political policy they are little different from those who embrace secular humanist ethics but maintain a belief in the supernatural. They are also quasi- or proto-Humanists.
I will end this post with a caveat. I am not trying to set myself up as the gatekeeper of Humanism, deciding who is in and who is outside the club. I believe that Humanism is self-selecting, that is, once a person has arrived at a certain set of core beliefs and values, one can be described as a Humanist whether one has heard of the term or not. Humanism is a descriptor for those who hold a certain set of values. That said, the opinions I express in this post should be seen as a starting place for discussion on the overlap in values reflected in Humanism and Progressivism.