the agony of thought

Sometimes a person seems to writing about one thing but turns out to be writing about everything. 

For instance, William Barclay was writing about religious faith here, but come on....

We tend to accept things because we are told them and to possess them merely at secondhand. If we undergo the agony of thought there may be much that we must discard, but what is left is really ours in such a way that nothing can ever take it from us.

Source

https://www.bibliaplus.org/en/commentaries/290/barclay-daily-study-bible-nt/1-corinthians/15/1-11