My question is, since the Christian God judges people, since people receive punishment or reward from God, doesn't that make "good and bad", "right and wrong" the domain of religion?
Only between you and GOD and most likely only after you're dead.
And no, good and bad are not the domain of religion.
Let me try to expand on that slightly because in some ways Christianity is quite different from the other two Judaic Religions in that regard.
Judaism and Islam are deeply involved in interpersonal relations. One of the funtions of both Rabbi and Imam is in settling disputes. When you read the Koran or Tanakah you find sura after sura, passage after passage, dealing with interpersonal relationships. If your brother dies you gotta bang his widow. You can charge interest to an infidel but not a believer. They go into very great depth on what you can eat, what you can wear, how you treat different classes, who you can or cannot associate with, how you should wear your hair, when and how to pray and on and on.
Christianity is quite different. Jesus boiled all the suras, all the passages down to the essence, Love GOD and love others as you love yourself. Christianity makes some basic assumptions.
The first is that you must love yourself. Often that last part of the two part commandment gets neglected, but it's important. GOD wants you to love and be happy with yourself.
Next, GOD assumes you're an adult and have some idea of what you like,how you like to be treated. Instead of a precise set of rules, good an dbad, right and wrong, he says, "Look, treat other folk the way you'd like to be treated". This is important so we'll come back to it in a second.
Finally, GOD says "Love GOD and show your love not by what you say, but how you behave."
Now let's head back towards the idea of a Christian supporting a Secular Government. Would the Christian like being forced to deny his Christianity, or pray to some other GOD? If not, if the Christian would not like to be treated that way, then GOD says you cannot treat OTHERS that way. For a Christian to try to force Christianity on others would be to violate the very commandments we were told to obey.
Aslan is not a Tame Lion