The Crucifix in Phasmophobia is a very important item. It can prevent the ghost from hunting twice before it will disappear, if placed in the right location.
A common mistake many people make with the Crucifix in Phasmophobia is to walk around with it in their hand thinking it helps. It doesn't. If it is in your hand it is useless.
The Crucifix is only effective when place on the ground or a table. In your hand it may look comforting, but it does nothing. Another thing to keep in mind is the protection radius of the Crucifix. This radius is crazy small. Except in small rooms, I always place two and often wish for more. In a larger room, they are nearly useless. In the largest of rooms the odds of getting them in the right spot is so low I don't waste my time placing them.
A Crucifix will only stop two hunts before it is depleted and disappears. At this time there is no indication that it has been used at all. The only way to know if a Crucifix has been partially used at this time is if you have the Objective to prevent the ghost from hunting with a Crucifix and to see it marked off on the whiteboard in the truck.
Of course, if the Crucifx disappears you know it's been used twice, and now you don't have protection. I usually place these as one of the first things I do after discovering the location of the ghost. Unless, of course, the size of the room makes this a waste of time. For small rooms I will often place one in the room and one outside the door to the room.
Note: if the ghost hunted and you had to hide, then the Crucifix did not work.