Paladin is the highest known rank of ability a Mage can achieve, second only to the Magic King. If two Paladin class magi battle each other it could put the world in danger. The next highest rank is Cardinal Class.
To achieve Paladin rank you must exceed or pass the Cardinal rank.
One example of a Paladin class technique is suppressing your own magic to zero and then unleashing it in an instant. (Source: Trinity Seven Chapter 15 the ENGLISH version).
According to explanation from Master Akasha to Arata in chapter 77
To begin with, "Paladin" is not simply a class to indicate strength alone. There's a bigger intention. We are called the "Arbiters of Heaven", but those Arbiters will choose the next generation in the turning point of the history. It can be said that "Paladin" are "God Candidates chosen by the Gods".