In his work The Theory of Moral Sentiments, Adam Smith speculates that what keeps us from have perfect affection for all around us is mortality. He defines affection as habitual sympathy; we come to care about those people with whom we spend a lot of time sympathizing. We just don't have the time necessary in this life to think about everyone well enough to care about them the way we should.
Scott Sumner ended a recent blog post with this idea:
PS. Suppose you had time to read 33 million 6 volume novels about each of those 33 million lives. Karl Knausgaard or Elena Ferrante-type novels. Do you think that might change the way you regard those 33 million people?Again, the idea that perfect affection is laudable, but unattainable given our current mortality constraints.
Zion requires us to be "of one heart and one mind," which can be interpreted as perfect affection. But our moral philosophers are telling us that perfect affection is impossible in our current state. I believe the solution comes through Christ. Instead of developing a relationship with each other human on Earth, I only have to develop one relationship with Jesus. He is outside time, and so has perfect affection for everyone. I can come to a point where I treat others with charity not because I have developed charitable feelings towards them, but because I understand Christ's love for them and I seek to reflect that. I will come to trust His judgment, and when He loves someone, I can then treat that person as if I love him, too, without having to take the time to actually come to love him.