For the Crucifix to successfully prevent a hunt, it must be in the vicinity of the ghost when it starts the hunt. The Crucifix has a range of 3 meters for most ghosts, and 5 meters for the Banshee. When the ghost initiates the hunt, it checks to see if it is in range of a crucifix. If it is, the hunt does not begin, turning all the lights off and taking one of two charges the Crucifix has.

If the ghost is out of range of the Crucifix (even if the Crucifix is located in the Ghost Room), it may initiate a hunt as per normal. No sound or animation will occur when the crucifix blocks a hunt. It will simply disappear once both charges have been used.

The Crucifix must be within range of the ghost's hunting spawn location in order to work. Because of this, the Crucifix isn't always reliable, because ghosts may spawn at a random, short distance away from their "center point" (ghost room). For example, if a ghost spawns for a hunt just out of range of a Crucifix, the hunt will proceed as normal.

If a crucifix stops a hunt, the Total Activity chart will show 10 very briefly and then drop sharply.
information from: https://phasmophobia.fandom.com/wiki/Crucifix