Increases healing by X% on friendly targets at or below 50% health.
Test of Faith is a talent in the Holy tree, you will need to spend 28 points in Holy to max it. It allows increased healing and spell criticals on the friendly target below 50% of their health.
Patch 3.1.0 (2009-04-14): No longer increases critical effect chance of your heals, but healing done on targets lower than 50% health is increased from 2/4/6 to 4/8/12%.
Edit
Patch 3.1.0 (2009-04-14): No longer increases critical effect chance of your heals, but healing done on targets lower than 50% health is increased from 2/4/6 to 4/8/12%. 
