His post was completely grammatically correct. He wasn't necessarily refering to the Christian god (which is a name, God), but he said 'there is a god,' which could refer to any gods. The word 'god' does not get a capital, just like the word 'deity' does not, only the name 'God' which refers to one particular faith's deity.
(BTW, pretty original to call your god, God. That's like naming him 'Deity.')