There were no dictionaries yet when "ghost" first came into English. But certainly back in THAT time, it was not limited to spirits of the dead. It also at that time referred to living spirits like angels, demons, and God.
You got the word wrong. It's Der Heilige Geist, not Heist.
There is no such word as heist in German, although there is the word "heißt" which is the singular third person verb that means "to be called" or "to mean"
The expresion Heilige Geist quite specifically means the Holy Spirit. In the original Old High German , the term for "the Holy Spirit" would be "der heilago geist." And by the way, you misspelled Heilige just like you mispelled Geist.
The word geist has a number of different meanings in German: spirit or ghost, mind or intellect, apparition, spirituality or inspiration, and humor/wit.
No it isn't. "Geist ist geil" is a German phrase that translates to "Spirit is awesome" or "Mind is awesome" You see, although geil CAN be translated as horny or lustful, it also can mean something as cool, awesome, or great, similar to saying "awesome" or "cool" in English slang.