Is it permissible for a Muslim to work as a guard in a church primarily used for tourism, and is it permissible for him to perform prayers inside it?

It is permissible to pray in a church out of necessity, though some scholars disliked it due to the presence of images.

However, working in the construction or guarding of churches, or similar activities, is not permissible, as it constitutes aiding falsehood. Therefore, it is not permissible for a Muslim to work as a guard for a church, and the earnings obtained from such work are not lawful.

A Muslim must search for another job. If the responsible party insists on keeping him as a church guard, this is an attempt to tempt him regarding his faith. He must leave this job and seek another, even if for less pay, for Allah has guaranteed provision for whoever fears Him.