It is correct to use both! However 'May god bless you' is more often used in greeting cards or as a bible quote,while 'god bless you' is used in normal speech.

I agree, we can use both, and it is used by many religions

Both are acceptable in different instances. "God bless you" (as an example) is used when someone sneezes, etc. "May God bless you" would be utilized as a personal blessing towards someone that is trying to accomplish a feat, etc.

Good. I think it is a pray and it should be "May God Bless You". It is quoted in most of the religions.