The original TFT system and all of the off shoots like GURPS or HOW, do not follow the D&D class system nor specifically the "cleric/paladin" class conceptions. These types of conceptions are left to the referee/players to work out and use if they wish.
The system is set up so that there are only two broad character concept generalizations: warriors and wizards. What type of warrior or wizard you make (barbarian, archer, ranger, druid, illusionist, necromancer, cleric) is left to you the player to conceive and develop. YOU have the FREEDOM to conceive of a spell casting barbarian, wizard duelist, or paladin as you wish
Religion or gods specifically are not a determining factor/limitation to any character skills/spells--unless you the referee/player want them to be.
A referee could say "these spells are only available to to players who are followers of this god" in their setting...or could say a "warrior who serves this god gets access to these spells at a cost of 1 instead of 2 pts. at creation" One could even go so far as to take specific spells and make them "blessings" for followers--an inborn or gifted ability such as "followers of this god can cure light wounds like the spell, no casting needed-just pay EN cost"
Creating a "religious class" is possible within the current rules. Turn the undead with a religious relic/holy symbol? --anyone can unless you as a referee decide to limit who can and who cannot. "Followers of the light bringer can turn undead with a blessed torch --but to all others it is a ordinary torch."
The spells typical of a cleric class are there for blessing, protecting, etc. but in HOW they are available (potentially) to anyone. You can decide to make them only available to followers of certain religions--if you choose to make it that way.
Unlike a more stringent class based system HOW is a wide open tool box. You get to decide for your game world if you want to create a special religious sect or cult with access to certain spells as you desire.
Want to turn the Bless spell into a native ability for all followers of a specific god? Want to make the Curse spell an "evil eye" for followers of a certain god? In HOW you have the FREEDOM to do so!
Don't be afraid of your freedom and in all cases do what is right for you, your players and your game to make it fun!