There is no limit beyond this to prove true religion. Fire is a nature and neutral. No judge can be better than fire. If a human is unable to burn in fire it is perfect that his beliefs and actions are correct and the God is only happy from his religion. This is not an easy task for a human to come forward just to prove his religion by leaving his family and pleasures of this world for the sake of God to guide all people to correct path without asking any benefit in return. I believe that Non-Muslim or false Muslim (Ahmadi etc) cannot come forward for this because their connection to God is not correct. That is why they do not believe that the God can save them if they enter in fire. I must also say that non-Muslims don't even have 1% faith that they can remain immune to burn in fire.
So the way we Muslim are ready to go in big burning fire to prove Islam as true religion without using any chemical on our body in front of world media, the same any non-Muslims or false Muslims (Ahmadi etc) who claim that their religion is true, then they should also come and get ready to go in burning fire with us without using any chemical on their body. Our goal is not to convert one person into Muslim, but to prove religion Islam as true religion to all religious leaders and followers of all false religions.
If any Non-Muslim is ready to go in fire then following are rules and methods:
1) Place of fire burning event will be a big ground far away from public living and the audience should be able to easily reach there
2) Setup at least 500 ft high woods so audience and media can easily watch from long distance
3) At least five Muslim and five Non-Muslim expert doctors should be present who can check body of people who are going in fire to test and verify that there is no chemical used on their body and issue a clearance certificate.
Once above conditions are done, three months' time will be given to participants to advertise about challenge in Muslims and Non-Muslims of the World.
Note: We Muslims are surely ready to go in fire. Why no Non-Muslim wants to go in fire with us? Don't you believe in your God and your beliefs and faith? If you don't believe to remain immune in fire of this world, how can you remain safe after death from hell fire? We don't ask any Non-Muslim to go in fire without us so any Non-Muslims have no right to ask us to go in fire without them. What we say is that Non-Muslims will go in fire with us so that result of true and false religions will be done through fire by the order of Allah Almighty in front of people of this world.