This elder (your uncle) reminds me of Jesus words in Matthew 23:15
Woe to you, scribes and Pharisees, hypocrites! for you traverse sea and land to make a single proselyte, and when he becomes a proselyte, you make him twice as much a child of hell[c] as yourselves.
You can tone down your response or simply don't say anything and make him wonder. You have full control and not any religious leaders. What I got from his letter is the fear and sadness he feels about sickness, death and complex problems that we cannot fix.