Paladins aka oathtakers gain their powers due to their commitment to a series of oaths. These can be to deities or their subordinates, but it can also be to temples, religious sects, countries, or even themselves.

This form of power from oathtaking was granted to all beings by Horkos, God of Oaths , but the great majority of oaths have little to do with this deity itself. Of course, breaking an oath is kinda a big deal, so be careful, as Horkos is also responsible for punishing oathbreakers. The oaths for paladins vary widely, and are seen in the subclasses :)

(see Pacttakers / Warlocks)

I will say, in normal dnd you choose your oath at level 3, which is a bit weird. Here, you still gain the benefits of your oath at that level, but you have to choose your subclass/oaths since the start!