An iron cross made of nails is recognized in a few belief systems. 
Noteably, in German, the Nagelkreuz von Coventry is a symbol of Peace & Reconciliation. 

Iron has throughout time been perceived universally as a symbol of strength, courage, honor, tenacity, and protection.

Nails themselves have ties in almost every major religion. For example, the crucifixion of the Catholic Christ which was both a violent tragedy while also a means to the salvation of humanity.

In more folk practices, a cross of nails is placed in the home as a protection to it’s inhabitants, and the physical space.

My tip is to place this cross over doorways, whether hung or just placed at the top of the door trip leaned against the wall.