Classes are leveled through their Equipment Level. Currently the in game system for equipment level is very simple.
For a Shadow Level 1 item you will get 1 Equipment Level, so on and so forth up until Shadow Level 5.
Radiant items give 6 equipment levels.
The max Equipment Level is currently capped at radiant, though the devs are hinting that a new way to forge rings is coming soon.
Players can reach a total of 22 Equipment Levels total, bringing the total character level to 42.
Pearls (previously Stars) do not affect equipment level anymore and Forge Level was removed from the game.