As an addendum to my last post on divine mercy, I'd like to put those ideas in more secular and universalist terms.
If a person realizes he/she has done wrong, then fixes the wrong, and resolves to do right, doesn't that turnaround come from the person and not necessarily from God? Yeah, I think that God equipped us with the faculties to figure out how to be nice to each other. Right living may not require prayer, per se, but at least some thought as to whether one's actions are good and not hurtful to one's neighbor.
But Jesus said that confession and communion are necessary to receive his mercy. Well, Jesus put his unfathomable message of mercy into accessible terms in the Catholic framework of Sr. Faustina and her peers. In the greater scheme of things, confession might translate to a repentant change of heart, and communion might translate to being a good person for others and ultimately for the greater glory of God (there, I sprinkled in some Jesuit goodness; did you catch it?).
Is belief in God necessary to be a good person? Eh, I wouldn't call those dependent variables. Different religions, different belief systems--even atheism--are just different paths up the same Meta-God mountain. I think that after all is said and done, living ethically and being a good person are what matters to Meta-God. The best we can do on this earth is to live truly to our hearts; leave the rest in Meta-God's hands.