What's the main idea of the poem?

In his harrowing two-part poem "The Chimney Sweeper," William Blake explores the motifs of poverty, faith, society, and manipulation through the eyes of child workers. The main idea of the poem, from my reading, is as follows: though religious faith helps people overcome the struggles of their own lives, it can also cause society to ignore the needs of others, and it can do more harm than good.
While readers may take issue with this notion, it's important to consider the context of the poem. The narrator lost his mother and was sold by his father, so he and other child laborers have nothing but the clothes on their backs and dreams of angels setting them free from life's problems. They are taught that if they work hard, they will be kept safe from peril—if not on earth, then in heaven—which brings them peace.

Though the morning was cold, Tom was happy & warm;
So if all do their duty, they need not fear harm.

The second part of the poem shows the consequences to this idea. It begins as the narrator finds a child—the chimney sweeper—whose parents have abandoned it in the snow to go to church; the text of the poem is largely from the child's own point of view. He reflects that joyous church rituals, like singing, make people think life is okay even though it isn't, and that his childhood happiness caused his parents to believe that he could stand a miserable life as a chimney sweeper. The poem's end implies that the members of the powerful strata the chimney sweeper's parents worship—including not only God but also his figurehead, a priest, and the king—are the very people who have earned vast wealth on the backs of child laborers.


They . . . are gone to praise God and his Priest and King,
Who make up a heaven of our misery.
https://www.poetryfoundation.org/poems/43653/the-chimney-sweeper-a-little-black-thing-among-the-snow

https://www.poetryfoundation.org/poems/43654/the-chimney-sweeper-when-my-mother-died-i-was-very-young