Holy Knights

The Holy Knights (聖騎士 Seikishi) are the most powerful knights in Britannia who are heroes that protect the kingdom. They are terrifying individuals, with each able to match an entire country's army in strength, and having magic that transcends human knowledge imbued within their bodies.

10 years ago
After the Seven Deadly Sins were framed for killing the Great Holy Knight, Zaratras, and plotting to overthrow the Kingdom, the entire order of Britannia's Holy Knights were dispatched to kill them. After the group of knights' supposed offense and subsequent escape of at least four of them, the Seven Deadly Sins were dissolved and were relentlessly hunted by the Holy Knights.

Present day
A few days before Elizabeth found Meliodas, the Holy Knights had orchestrated a coup d'etat and captured King Leoneth, the king of Britannia. The kingdom, now under the control of the Holy Knights, fell into a state of distress as citizens from towns and villages surrounding the kingdom were enslaved or forcefully recruited into joining a furtive war effort; the men being forcibly trained as soldiers, the women and children being forced to amass food for storage, and the elderly being forced to construct castle walls. Death has now become the penalty for insubordination under the rule of the Holy Knights.

It turns out that the reason the Holy Knights betray the king was to prepare for the Holy War and the king say to put down their sword so that the kingdom wouldn't get into the war but Dreyfus and Hendricksen betray the king to get ready for war.

Great Holy Knight
The Great Holy Knight is the highest and the most powerful of the Holy Knights. There strengths and capabilities are still unknown but they appear to have phenomenal powers much greater than regular Holy Knights power. They are the leaders of the Holy Knights and has the authorities of commanding over them. There are only three known Great Holy Knight in the series so far, and it is suspected that there are usually only one of them per era.

Holy Knight
The Holy Knight is a title given to well established knights that have strength close to thousand of armies alone. They have command over ordinary knights and train apprentices to become full fledged Holy Knights. They are all imbued with extraordinary powers, such as being able to manipulate lightning like Gilthunder.

Apprentice Holy Knight
Apprentice Holy Knights are disciples of existing Holy Knights who are training to become seasoned and qualified Holy Knights. They are still fairly powerful and has command over ordinary knights, however, they are still far from reaching the abilities of a Holy Knight.

New Generations
The New Generations is a title given to Holy Knights who have suddenly appeared with extraordinary powers. The powers are obtained from drinking blood from the Unknown Demon, which has the abilities to change an extremely weak warrior into an immensely powerful one. However, only compatible knights will be granted greater strength, or they will merely explode to death like Twigo. The New Generation was first mentioned by Glithunder.

Trivia

 * Holy Knight is a special job class in Final Fantasy Tactics for the PlayStation.