Requirements
25 Faith is required for joining. The requirement, however, can be lowered by 5 for each area boss cleared while being summoned as a white phantom.
For example, if 4 bosses are defeated as white phantom, only 5 Faith will be the new requirement (25 - (4x5)) to join the Warrior of Sunlight.
How to Join
Pray at the Altar of Sunlight, which is located next to the bonfire below the Hellkite Dragon in the Undead Parish