From Diablo Wiki
The Zakarum (also known as the Church if Zakarum or the Church of Light and its followers the Children of Zakarum) is a religion that was powerful and prominent throughout much of Sanctuary during the centuries preceding the events of Diablo 2. The Paladin class was a sort of Holy Knight of the faith, fighting for justice and using the holy powers to defeat evil and especially demons wherever they were found.
Unbeknownst to the Paladin or the other heroes, centuries ago the Soulstone containing Mephisto's essence had been hidden deep in the Zakarum's catacombs below Kurast. Much as Diablo did in Tristram, Mephisto was able to send thoughts and influences out from the soulstone and thus influenced and corrupted the Zakarum high council. Ultimately they set Mephisto free, and his power mutated the humans into bestial demons that the hero must defeat in the later portions of Act Three.
Though the Zakarum priests fell into demonic evil in the Kurast area, Zakarum remains strong and a force for good throughout many other portions of Sanctuary. Diablo III updates the story of the world and the changes that took place for the Zakarum over the 20 years since Diablo 2. Reaper of Souls introduces the Crusader class, a sort of special-forces warrior for the Zakarum faith.
Zakarum and Faith
The Zakarum have had a long history of battles with demons, sometimes assisted by Tyrael. The faith is not devoted to angels, though. In January 2014 Blizzard CM Nevalistis explained the relationship leading up to events in Reaper of Souls.
This spurred the thought that humans were powerful beings of Light, and Akarat set out to spread the word that this Light existed within each human.
It's not so much that Zakarum is the worship of angels and the Light specifically, so much as it is the ideal that this Light exists within each human and humans should seek to acknowledge their "inner light" in order to live good lives. The sight of what was interpreted as an angel spurred this belief, and the acknowledgement of Light as a concept is certainly important, but neither are really a central focal point for worship.
The Zakarum do, generally, look at angels as "the good guys," but that's more of a jump in logic than part of their beliefs. Demons are bad, angels fight demons, so therefore angels must be good.Malthael, on the other hand, appears to be spending much of his time in Reaper of Souls involved in an extensive PR campaign to clear up some of this confusion about the relationship between man and angels, so we may see this change.
The Zakarum religion was started well before the mid-twelfth century based on the visions of Akarat. It is said that he started the religion after the visit of Archangel Yaerius in his dreams. These visions most likely included some sort of divine intervention, or possibly a demonic one. The book Visions of Akarat is likely the main holy writing for the Zakarum, but it's also possible other chronicles of Akarat's life is in existence.
The Church of Zakarum gained prominence in the East, and the leaders of the Church decreed Akarat's visions would be spread throughout the known world in order to redeem the masses. Thus, the Church selected a group of its most charismatic and devoted priests and sent them on a mission to proselytize the people of the West.
Unfortunately the Church had not prepared these men for the rigours of travel or the hazards of the world. Those priests who survived their missions recounted tales of harsh weather, inadequate supplies, attacks from bandits and even encounters with horrible monsters.
To ensure the success of future missions, the Church set about training holy warriors, Paladins, to accompany and safeguard their missionaries. These "Protectors of the Word" proved to be more successful at converting the native peoples than the Priests they were assigned to defend. Impressing the locals with daring deeds, powerful weapons, and martial prowess was far more convincing than the condemnations of a soft-spoken monk. However, once the Word had been spread to every major city of the West, the "Protectors of the Word" faded from public view.
Some decades later, Paladins were again called into service. During the height of the Time of Troubles, the Church commenced a second campaign of conversion. This time, however, the unconvincable were deemed to be evil. The Zakarum Inquisition spread through the lands like a tempest, laying waste to all suspected of demonic possession or corruption. Leading this crusade was a new generation of Paladins, known as the "Hand of Zakarum". These cavaliers of righteousness swept through the lands, expunging the taint of demonic contamination wherever they found it.
In the midst of this bloody crusade, a rebellion arose within the ranks of the Paladins of Zakarum. The rebels condemned the methods of the Inquisition, proclaiming that their new Order of Paladins should protect the innocent, and that the evil corruption they fought was merely evidence of their forebear's failure. They resolved to fight the true source of corruption, the Three Prime Evils - Diablo, Baal, and Mephisto. And so, these rebellious Paladins left their Zakarum brethren and ventured west.
Paladins use holy magic as gifted by the High Heavens. They must maintain strictly ordered lives, constantly upholding the cause of Virtue and Light. They must never succumb to worldly temptations lest they risk being deceived into following "false lights" — demons masquerading as Heavenly beings.
Paladins may use their skills to increase their prowess with sword and shield, as well as lend blessing "Auras" to themselves and to any who join them. They are particularly effective against the undead, as they know many holy incantations effective against these types of creatures.
There are no detailed accounts on exactly how the Church of Zakarum is organised, but at the time of 20 years ago it was lead by a High Council, who governed the Church. A Que-Hegan, also known as a Great Patriarch lead the Council. There might have been additional ranks to the Church, but somewhere under the High Council there was Arch Bishops, Bishops, Priests, Priestesses, Paladins, nuns and monks.
Of course, this was when Sankekur was the leader of the Zakarum, and embodiment of Mephisto, so they might have moved away from this.
Priests and priestesses of Zakarum don't practice chastity vows.