I've read a lot of misinformation on this thread so I guess I'll offer my own research into the piety gaining traits.

First, some general info about traits. Traits function in levels. The first level of a trait is pretty easy to gain, but to gain additional levels you need to do more.

Relevant traits, their levels and how I've noticed them to be gained:

Respected by Pope (+1 Piety, +1 Command vs Muslims)
Adored by Pope (+2 Piety, +2 Command vs Muslims)
Exalted by Pope (+3 Piety, +3 Command vs Muslims)

This trait category seems to be gained by the faction leader if you do several crusades during his reign (dont need to be led by him). The first level needs at least two crusades during the same reign but so far three crusades is almost full proof.

Crusader (+1 Piety, +1 Command vs Muslims, +1 Chivalry)
Great Crusader (+2 Piety, +2 Command vs Muslims, +2 Chivalry)
Grand Crusader (+3 Piety, +3 Command vs Muslims, +3 Chivalry)

Acquired by participating in a crusader. Leading a sucessful crusade (meaning you actually take the target city) almost always gives you the third level.

Conforming (+1 Piety)
Reverend (+2 Piety)
God-Fearing (+3 Piety)

These seem to be aquired by sticking a priest with the general's army IF he doesn't have any other religious traits. If you have ANY other religious traits you will not gain this one. You can gain the others later though and will still keep this one. My SOP when a family member comes of age is to stick him with a priest at least until he gets the conforming trait.

Growing Conviction (+1 Piety)
Strenghtened Resolve (+2 Piety)
Feeling Invulnerable (+3 Piety)

I've only seen this trait category being acquired on one campaign so I don't have enough research data. The growing conviction one was gained after an Heroic Victory against Muslims with the English. I gained the the 2nd Level a few battles against the Moors with the same general and the last with another Heroic Victory against Egypt with the same general where he was caught alone but since his bodyguards had 2 gold chevrons he managed to rout the enemy army single-handedly. Like I said, not enough data to confirm the triggers.

Holy Soldier (+1 Piety, +1 Command vs Catholics, +1 Chivalry)
Holy Defender (+2 Piety, +2 Command vs Catholics, +2 Chivalry)
Holy Warrior (+3 Piety, +3 Command vs Catholics, +3 Chivalry)

These work exactly like their crusade counterpart.

Religious (+1 Piety)
Dutifully Religious (+2 Piety)
Religiously Active (+3 Piety, +1 Chivalry)
Devoutly Religious (+4 Piety, +2 Chivalry)

These are gained as governor both for building church line buildings and priests. The first level is very easy to gain, the others are increasingly hard. The only ever general I managed to get to the 4th level was the governor of antioch who crusaded there when he was 17. He built antioch all the way to Huge Cathedral, had a Theologian's Guild HQ there and must have recruited in excess of 10 priests before he died a peaceful death. Adding all his traits/retinue he had 17 Piety and yes, the game does seem to keep track of levels higher than 10 as I kept seeing admin bonus changes in the city whenever he gained a point over 10.

Servant of Heaven (+2 Piety)
Dramatic Visions (+3 Piety)
Visions of Prophecy (+4 Piety)

I've only seen this trait category ONCE. The general was born with the first level and gained the others as governor of the city where he was born. He was the son of the general described above, was born in Antioch when the city already had a cathedral and a master theologian's guild. No idea how to trigger it. Might be bloodline related (son of a pious general type of thing).

In addition to traits there are retinue and items that add to piety but myself and a few of my friends who are also playing have been focusing and sharing info on traits only.