It’s a reference from the book of Daniel 3 to the Israelites in captivity who stood against the king’s order for everyone to bow down and worship the idol he had made or be thrown into a fiery furnace. Bow down or die. But when three Israelites refused and were thrown into the furnace a fourth man “like unto the A son of the gods” appeared with them and they came out unharmed.
“...three Israelites refused and were thrown into the furnace a fourth man...”
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https://www.youtube.com/watch?v=ewY9U_FVPeM
Re: the fourth man in the fire
He has been with me in the fire many, many times. He will protect me as long as I stay obedient to His will, His Word, and His ways. No fear.