I often see it said that the quote, "If there is a God, he will have to beg my forgiveness" said that it was carved into the cell in a WWII concentration camp, is there any truth to this?

Someone has asked about this on /r/quotes before (link).

The user /u/sonic-servant posts links to blogs/personal journals which attest to the quote in the cell wall. /u/tolos linked to this book which refers to the quote also.

I don't know if this is overwhelming evidence, it might be worth contacting the relevant museum directly if possible (according to this blog the site is the Mathausen Concentration Camp in modern Austria).