Notice who doesn't suffer it -- Sallah and Henry Jones, Sr. For Sallah, the Grail holds no spiritual meaning
….until he sees its healing power on Henry.
Raiders established Sallah as at least understanding and respecting the Christian religious artifacts--the Ark in that case, with the following--
Sallah: "There is something that troubles me.."
Indiana: "What is it?"
Sallah: "The Ark. If it is there at Tanis, then it is something that man was not meant to disturb. Death has always surrounded it. It is not of this earth."
So if he's not a full believer in the religion (as in follower), he's certainly open-minded to the possibilities of what each represented, hence his reaction to the Grail's healing power.
The desire to possess the Grail, the "Grail madness," suggests that Indy and Elsa were never worthy of the Grail or the task of guarding it like the Crusader knight. Indy's mantra is right up front in the film -- "It belongs in a museum." The man in the fez in Venice asks Indy why he seeks the Grail -- "Is it for God's glory? Or for yours?"
I'm not sure that for Indiana, its a matter of worthiness; one of the running themes of the original IJ trilogy is that Indiana's skepticism always had to be smacked into the reality of the religions or artifacts he was dealing with; in
Raiders, he was on the fence about the Ark until its power was revealed in front of him (notice how he warns Marion to close her eyes--which he would not do unless he believed to some degree), and in
Temple of Doom's conclusion, of the
sivalinga stone(s) he admits--
"I understand its power now."
Unlike Elsa, he actually had a faith to rely on in selecting the right grail and taking a drink from it; it was not based on a strict, clinical assessment of artifacts. After two films in, and debating with his father on the way to Berlin in TLC, Indiana had to finally reach into himself--his true beliefs, which made him worthy. Elsa was all about the potential of power (and not in the half-believer way of a Belloq) and glory of it all, so she--like Donovan--were not worthy of its benefits (or the job of protecting it), even it she knew the real Grail on sight.