I must push back a little against your conclusion that “It is the duty of all decent people to be thankful” is not “contrary to Christianity, but it is certainly not Christianity itself.” Your statement could be read by certain hyper-Calvinist types as implying that indecent people have no duty to be thankful (because they are predestined to damnation).

We must be crystal clear. All people whether decent or indecent are without excuse for their failure to give thanks to God (Romans 1:20-21). The same law that condemns the decent condemns the indecent (Romans 3:9). God does not want anyone to perish but it is His manifest will that all people come to repentance (2 Peter 3:9).