What would you consider to be evidence for God?

[Angrboda]

Math may just be a concept of the mind.

It's a bit more complicated than that. To say something is simply a concept in the mind does not account for its objectivity. How is it that various people can come to complete agreement with respect to some concepts like the nature of mathematical objects. I think most people would say that mathematics is objective. But in what way is it objective?

Something is objective if it exists and could be known independently of who knows about it or even if no one learns anything about it at all.

The problem with this is that there's no way to demonstrate that what is known is independent of the nature of the knowers. We're all human and so we may share certain cognitive realities that are universal within the species, but yet are not truly universal. We can't be sure that mathematics is independent of the nature of the knower, so we can't simply conclude that it is objective. This is a systematic flaw with your definition of objective when applied to concepts.