It's sorta creepy how Nagisa and Honoka know exactly what to chant in order to transform, know what their transformed forms are called ("Cure Black" and "Cure White"), and automatically attack in unison--it implies that their consciousness and will have been taken over by the Power of Light. In fact, when Nagisa says "Now that you mention it...I did say something like that a moment ago" (ch. 1), it all but confirms that fe hadn't been speaking of feir own volition. Obviously, this is just weak storytelling with the author being too lazy to include a training montage and not meant to imply anything sinister; we're just meant to accept that the forces of Light are righteous because the author says that they are without thinking about it too much.
That's kinda the impression I get from a lot of this series--that you're not supposed to think about it too much. Just enjoy the fight scenes and declarations of friendship and the strength of banding together, with maybe a tad of romantic yearning sprinkled in. They're fighting against the Dark King and feir followers in order to get back the stolen Prism Stones and restore the Garden of Light to its previous beauty, but what they're really fighting for is their friends(!) The fight scenes aren't actually all that interesting, since they all end with the same move: the two of them holding hands and shooting intertwining beams at their opponent.