If it's heresy, then why did Jesus endorse it by praising Thomas addressed him as "my God"? Then there's the small matter of Him saying "Before Abraham was, I am." The Jewish authorities knew what he meant, and they got even more worked up about it than you did, for that very reason. They were ready to stone him.
If Thomas calling Jesus god, then what about when Jesus called Peter Satan? I'd think that Jesus knows more than Thomas.
Look up the figure of speech called 'hendiadys' and then you'll understand about Thomas.
The Jewish authorities would have known he was mad and insane had he claimed to be God Almighty.
However, the possibility that Jesus could be the messiah, the Christ, the one sent from God was a real possibility and for that they hated him.
I guess that Jesus also made monkeys out of them through scripture and their own words didn't help matters!