In trying to understand progressivism, and what 'motivates' it; I recall my own time as a progressive, and it seems like the whole business boils-down to optimistic fatalism.
For a progressive there can be no root in the past, because there is an expectation of change.
For progressives (liberals, leftists) in the Christian church there is a rejection of Holy Tradition, a rejection of Scholastic logic, a rejection of Hierarchy, a rejection of the inerrancy of Scripture... all these are rejected, and Christianity is declared to be open-ended and to move with the times and reject the past - and so on.
But, if Christianity can change, what keeps it Christian?
There are those who say it is a complex mish-mash of all the above, so complex that the inter-relation cannot be pre-defined; and that view is coherent, albeit very difficult to evaluate and tending to deliver over Christianity into the hands of academics (which must be a bad thing).
But the mainstream of reformers in the modern Christian churches, almost all of the church leadership and the majority of self-described Christians - do not ever put forward a description of the basis of their changes.
They never state that basis upon which there 'reforms' are based.
I mean, traditional Orthodox might point first to Holy Tradition, Roman Catholics might point to the Magisterium, or catechism; Evangelicals would probably point to Scripture (in its plain or 'literal' meaning) - but the mass majority of progressive self-described Christians do not point to anything: they do not point at all.
Progressives do not reason from a basis, instead they reason from how things are, and how things are shaping-up to become.
(So-called) Liberal Christianity is therefore the process of adjustment of Christianity to how things are and how things are shaping up to become.
Is this good or bad? Well, the faith of Liberal Christianity is that it is good.
What is and what things are shaping-up to become is good; thus Liberals are optimistic about change; change is seen as getting better, change is seen as leaving-behind evil - as progress.
Thus, change is progress - change is always therefore called reform; and progress will happen, and progress is good: hence this is optimistic fatalism - because (for Liberal Christians - a Leftists in general) what will be, will be and that is how it ought to be.
To fight what will be will be is not just futile, but evil - because what will be is (by definition) better than what was; and traditionalism is evil because what was, is precisely what we are superseding.
Liberal Christians moral force is therefore an act of faith in what is and what will be; their moral disapprobation is restricted to those who regard what is and will be as evil: this is seen as perverse, but also in and of itself wicked: perversely wicked.
But what direction does this progress take us, where will we end-up, will that be good?
None of these questions can be answered.
Rather, the belief is that we will continue in the same direction - which has been good, and will continue to get better (why not?).
And if it does not seem to be better, then that is simply because we ourselves have been superseded and ought to be replaced by something better.
Is that a good thing? I mean, that those who cannot or will not 'get with the programme' are crushed or discarded? Well, yes, because the programme is primary.
Progressivism IS the programme.
Liberal Christianity is the open-ended process of moving Christianity to the Left - and since Christianity has been uprooted from tradition, scripture, authority and reason - then Christianity is what progressives say it is - and the only non-Christians are those who disagree with what progressives are today proposing as the next inevitable change.