The Sin player only initiates combat through reactions.
I have found it best to save these card for when I react and am attempting to get a kill.
you can only play those cards usually on the beginning of the FAITH's turn however,so a bit of planning and luck will be needed
Yep, start of a players turn for most of the cards. It's a question of setup, and trying to focus on the hero you want to kill, making sure they cannot move where there is no monster, allowing you to react, move another in, and drop an acolyte on them.
Certainly, if they have the option, they should move out of monster occupied spaces.