Romans 3:3-4:

For what if some did not believe? shall their unbelief make the faith of God without effect?
God forbid: yea, let God be true, but every man a liar; as it is written, That thou mightest be justified in thy sayings, and mightest overcome when thou art judged.

See also:

Let God Be True and Every Man a Liar
What Do I Do If I Don’t See Any Improvement After Praying?