Much of religion revolves around justice. Christians believe in the final judgment day when the evildoers will be punished and the righteous will be rewarded. At the same time, forgiveness and mercy are also at the very foundation of the Christian faith. Justice and forgiveness seem to be incompatible with each other. Atheists exploit this conflict to make a point that Christianity is immoral.
I think, the contradiction is in the reciprocity of these concepts.
An eye for an eye leaves the whole world blind.
This expression attributed to Gandhi, but, perhaps existing long before Gandhi, shows the flaw in the biblical concept of justice. “An eye for an eye” approach creates a vicious cycle of violence that can only be broken by forgiveness. Forgiveness and mercy, on the other hand, are supposed to create forgiveness and mercy in return. But sometimes they don’t.
Reciprocal relations have to start small and build on themselves. When I don’t know if I can trust a stranger, I trust him with something small. Then I build my trust on my experience. Trust builds on trust, but it starts unconditionally. It justifies itself, like faith. Or, rather, it’s justified by the experience of practicing it. In the same way, we can find out if forgiveness is worthwhile.