Christians believe that the Universe was not an accident, but was created by God, and that for a time, He came to earth in the person of Jesus Christ and lived a perfect, sinless life; a life, filled with love, that we should all emulate.
So where does a Christian's ability to decide what is right and what is wrong come from?
1. Our God-given internal sense of what is right and what is wrong; that is, our conscience. Physically, this is part of our brain, something programmed into the genetic make-up of every human being, and therefore, able to be passed on from generation to generation. It was there in Adam, and is present in every human being living on the earth today. That is why every human being, not matter where they live on the Earth knows it is wrong to murder, wrong to steal, etc.
2. The edicts in the Bible, principally the 10 Commandments and the Golden Rule.
3. The Holy Spirit; that is God who comes to live within us when we come to have faith in Jesus Christ and trust in Him to guide us through life and follow His perfect example of love and sinless living.
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Unfortunately, of course, every human being sins, and therefore falls short of the standards that God wants us to live by. But that doesn't make those shortcomings right, or justify the horrible atrocious that have been committed in the name of Christianity. When we follow Jesus, we are granted forgiveness for committing those sins- but we are also warned to repent and sin no more; we are not to use Jesus's promise as an excuse for continuing to sin; quite the contrary.
But the combination of our conscience, the Commandments, and the direction given us by the Holy Spirit gives us an absolute standard; a perfect example of how we are to live our lives. If we could all do that, the world would truly be a much better place to be.