If you hold it in your hand and right-click, all living creatures near you will disappear without a trace. It's impossible to craft the Cross; it can only be obtained in Creative mode (for now). When you hold the Cross, you gain a new effect:
God's Protection - it keeps your health at maximum, but you can die from strong hits.
Hallelujah.
Currently, the mod only has one item and effect, as well as a couple of sounds. After removing a mob with the cross, particles appear in its place. The cross can remove any mob, and it doesn't kill them, but rather removes them, making it more powerful.