3 ideas
In pietist Calvinism, salvation is predestined but a believer can only infer it from an inner experience of conversion that no one can verify. Because no outward act counts as proof, the doubt never resolves. Each sacrifice or period of self-examination only postpones the anxiety, so the believer's demands on himself escalate without limit.
The traits that make Hans admirable are the same ones the sect takes hold of: his tenderness, his attention to living things, and his longing for beauty and meaning. Someone like this is recruited through his depth rather than his weakness, because the group offers a bigger object for a devotion he already has. The practical warning is that vulnerability to totalizing belief can track a person's goodness, not a lack of judgment.
Hans never sets out to hurt his family. He is a loving man whose attention is simply pulled toward God and the brothers. The novel shows that his wife and son experience this redirected devotion as abandonment, just as if it were cruelty. The damage comes from the attention withdrawn from them, not from any hostility.