Dualism is the nature of existence. And often times, opposites do compliment each other.
In the beginning they theorize that there was a pure energy that created a hot plasma soup of matter and antimatter. When those two collide the result is pure energy and it takes pure energy to create them. All things are from one pure energy but there is a separation, transformation and recombining of energy in different forms and levels to create the observable universe. Things of mass are only denser forms of energy. So I wouldn't call existence about dualism as much as it's about energy given
structure and meaning through
symbolism. Matter and antimatter could have cancelled each other out in the beginning but for some reason there was a slight variation that favored matter.
There probably is no devil and is what we call God when we don't get what we want or get disciplined by fate and nature. In the Lord's Prayer it doesn't say "don't let satan lead us into temptation" it says "Lord lead us not into temptation". The bible basically has two messages, one is of hope, faith, truth, love, light, compassion, salvation, freedom, forgiveness and life. The other is of death, pain, lies, darkness, ignorance, judgment, slavery and doubt. That's more about choices, enlightenment, acceptance, maturing and belief than any perceived dualism. Take the good message, let love enter your heart, which doesn't mean be naive of those (all of us) that are lost or imperfect, but simply understanding. And know we have to
learn by our mistakes- (evil, darkness, lies, errors, etc) and successes- (good, knowledge, wisdom, productivity, honesty, etc). In the end they all serve the same purpose, which is for us to grow into who we're going to be.