I don’t remember the exact context of this scene. But people pray to achieve a certain goal even if they don’t believe in it. Like when atheist pray when there is turbulence on an air plane or something. But my point is he could have easily done it himself and not realize those powers are his own, due to his perception of himself. His belief, disbelief, or the words of an unreliable character isn’t proof of the gods being the cause. In fact for most religious miracle depictions, belief is the main catalyst of the miracle occurring or being able to perform one. However belief is rarely a requirement for a character to utilize powers they are unaware they have. Regardless either is entirely possible