One criticism laid against Christians is their use of circular reasoning to prove their points. We say "the Bible says" to prove God exists, which is His Word, and those opposes say we they don't value that as evidence and ask for an outside source. If I want to prove I exist, won't I tell you? What kind of outside source could you go to? If you ask someone who live ten miles away, all he or she can point to is my name in the phone book or what comes up in an online search. That is not direct proof of me but only evidence for me.
The first part of Ezekiel 12:25 says "For I am the Lord. I speak, and the word which I speak will come to pass". Of course it is circular reasoning to use God's Word to prove God. He speaks to us there so we tell people what He says.