Personally, I'd do it as a feat.
ARMOR OF FAITH
Prerequisite: Must be able to cast one spell
When wearing no armor and not holding a shield, your AC is equal to 12 + Your Wisdom modifier
When wearing light armor and not holding a shield, you can use your Charisma instead of your Dexterity to determine your AC.
Since a 1st level PC in 4th edition is a 4th level PC in 5th edition, it's should be a fair trade for a Feat. The DM could also trade away your medium and heavy armor for the feat. The truth is MAD is the only problem.
ARMOR OF FAITH
Prerequisite: Must be able to cast one spell
When wearing no armor and not holding a shield, your AC is equal to 12 + Your Wisdom modifier
When wearing light armor and not holding a shield, you can use your Charisma instead of your Dexterity to determine your AC.
Since a 1st level PC in 4th edition is a 4th level PC in 5th edition, it's should be a fair trade for a Feat. The DM could also trade away your medium and heavy armor for the feat. The truth is MAD is the only problem.