Hey guys and gals! Here is my 100% Achievement Guide for Infliction!
So, directly after you get the "Bright light, Bright light" Achievement (For killing the ghost in the 1980s), you are required to go through a painting of a cabin and be transported there.
Once you have spawned in, immediately to your right is a lone tree, walk up to it, inspect it closely and you will see a hanging crucifix from it. Once spotted, the achievement should unlock.
Go to 32:30 in my video to see how and where this unlocks.
Hope This Helps!