They will not believe me He means, they would not take his bare word, unless he showed them some sign. He remembered how they had once rejected him, and feared it would be so again.
Related Commentaries of Exodus 4:1
Exodus 4:1
1 And Moses answered and said, But, behold, they will not believe me, nor hearken unto my voice: for they will say, The LORD hath not appeared unto thee.