Apostles are those that are directly anointed and appointed by the Gods themselves, while the Paladins are members who are appointed through training and acknowledgement from the high ranking members of the church of the nine Gods.
That is the existential difference between an Apostle and a Paladin.
One is directly appointed by the Gods, and the other, by mortal being who are in charge of the organization created to serve the Gods.
If in a case that a Paladin member ends up being chosen by one of the Gods to become an Apostle, that Paladin would first have to relinquish their position as a Paladin in other to become an Apostle.
There is not a case where one can be both. It is either a Paladin or an Apostle.
It should also be made aware that the difficulty level attached to becoming a Paladin is not in the same category needed for one to become an Apostle.
