defining justice

I don't understand why Socrates cares so much about defing justice. I mean he builds a city just to define justice. I mean as a Christian I am aware that I cannot fully understand God, I just have to have faith. So why does he feel it is so important to define and understand the just and unjust. And who is he to declare that.I feel like the time he took just to define a term that cannot fully be defined is crazy. I don't know It may just be me, but people do spend their life's trying to define and understand God in the same since. To me justice is a word that is defined differently by everyone, and ultamately God has his own definition of just and unjust. It is just so confusing to me why it was so important to Socrates to define justice. To each his own I guess. Ps. I commented on Amanda's