Mine eyes have seen the glory of the coming of the Lord; He is trampling out the vintage where the grapes of wrath are stored; He hath loosed the fateful lightning of His terrible swift sword:

Which itself is adapted from "John Brown's Body." If you know anything about John Brown, he was a very religious man, and one of the main ways he got people to follow him on his anti-slavery crusade was by appealing to religion. In fact, as the Civil War went on, even the usually secular Abraham Lincoln began including more religious language into his speeches. This was partly to help calm the nation down due to the immense deaths during the Civil War. But it was also because John Brown's rhetoric was very effective. There were a lot of people in the north who actually did not want slavery to end (like McClellan and his moderate faction), because it materially benefitted him. There was actually very little material benefit to abolition for Lincoln directly or really most abolitionists. Abolition was an ideological project, and one which could not have been accomplished in the mid-1800s without using Christianity as a justification. In fact, Harriet Beecher Stowe's brother was the most famous preacher in America during the Civil War, and he constantly preached propaganda against the south and slavery which many people in the north agreed with, again, not because it materially benefitted them, but because they agreed that slavery was immoral from a religious standpoint.

One of the primary sources that Martin Luther King learned rhetoric from was the Bible. He was a reverend, and he sharpened his public speaking skills through the church. America has a long, long history of progressive rhetoric intertwining with religious themes. In fact it Steinbeck was using an appeal to religion in the passage you said you liked, even though you might not have noticed. So yeah, maybe there is a reason that the Bible is literally the best-selling, most widely read book in the history of humanity. Maybe it has some good rhetoric in it! And maybe people on the left can learn a little bit about how to capture people's attention and empathy by learning from its prose.