A mark on the forehead and the hand is sort of the anti tefillin. A mark on the forehead represents what is foremost on the mind which also marks, binds, the hand to its actions.
If you worship a false god that became a human sacrifice and if you wear that belief like a frontlet, you will persecute, criminalize, and marginalize unbelievers with extreme prejudice.
It is a choice to accept or reject evil. Most people just don't have the balls to reject evil.
And why do you speak as if it was a thing of the past? I can see the mark on your forehead.
Or maybe it was just Roman coin.