I agree that individualism and collectivism is one difference. But collectivism at one level means "we are all in this together", instead Progressive Liberalism positions identity groups as the oppressors and oppressed. That divisiveness has generally been used as tool to create a power struggle. It is pretty hard for society to advance when it is at one another's throats, but it is pretty easy for individuals to advance in such an environment.
As someone whose immediate family stretches over 4 races, we view the attempt to appeal to us based on the color of our skin both offensive and troublesome. And yet when the media and politicians both resort to fighting over what defines "blackness", you know this is not so much about collectivism, but about "us vs them".
I think Marx's greatest mistake arose because he never saw his vision of society expressed, it was all theoretical. People don't act like one big family and so they rely on a governance structure. I have used the term "kinship" as a vehicle to describe altruism people feel towards others and observed that the more diverse the society becomes the less "kinship" is evidenced by society towards other members of that society.
Marx observed corruption among the wealthy and somehow thought "power" among government would be less corrupting. That has not proven true.
Capitalism harnesses the power of "selfishness" to create a prosperous society. However, because as noted people can be selfish to the degree of corrupt and abusive, we need government to regulate abuse in the economic system. However, to regulate abuse, it can't be part of the system. In socialism, the government becomes part of the economic system, which is akin to putting the fox in charge of the hen house.
I do not subscribe to secular humanism for similar reasons. I thread the divide between recognizing God and the ultimate ruler of the universe and his command that we should care for his "Garden". I agree with the Deists, that the Creator endowed us with unalienable human rights, that no secular power has a right to deny unless doing so is to protect others from having their rights infringed upon.
I also believe however you want to express it, that every human is sinful. And that you can't create a society based on "human nature" that won't become corrupted and rot. The ultimate end of a secular humanism is moral collapse in the form of hedonism and debauchery.
Like children who act better with parents who set parameters, humans behave better when we embrace that the limits on our behaviors are not set by us, but by someone far greater than us.
The biggest problem with religious people is not their beliefs, but rather their attempts to force others to conform to their beliefs rather than tolerate those who hold different beliefs as long as their behavior does not extend to imposing behavior on other individuals.
A simple example is what consenting adults do in the privacy of their bedrooms is their right to do. But any society that promotes sexual promiscuity is doomed. Humans simply don't operate well under that model of behavior for lots of reasons not the least of which is the violence such behavior elicits and the lack of responsibility towards raising offspring needed to raise a baby to an adult.
