This is located in the “Glowing Sea”, a highly irradiated area to the SW of the map that has constant radiation. The church itself is found outside the boundary. The only way inside the church is through the roof to reach the steamer chest inside containing a huge amount of ammo and some weapons. There's also a deathclaw that may appear right after you entered the church. To get out, you have to use the fallen tree trunk as ramp.