My initial thoughts and looking forward to feedback:
Messiah will be a white mage, somewhat fragile healer
Move Set: Covert Witness - % chance to convert an enemy creep to join his side each time he heals an allied creep. Cap on max creeps converted. Creeps converted this way get bloodlust for X seconds. When bloodlust wears off, they have % chance to become an enemy again whenever an allied creep dies.
Sermon - Heal over time all units within small area around Messiah
Influence - A tally. When enemies die around Messiah, it increases. When allies die around Messiah, it decreases. All other spells are effected by inluence.
Blind Faith - When activated, all nearby allies attack faster, deal damage over time (called corruption, mechanically = to poison), and have a % chance to stun enemies for x seconds.