> I will lose the credibility

There's grammatical mistakes and then there is sloppiness. Only the second makes me disregard someone's comment.

> I will lose the credibility of my message if there is too much mistake...

The correct way to write this is "if there are too many mistakes", because mistakes are countable and plural. And it's fine to make grammatical mistakes if English is not your native language. You can only get better by practising :-)