Secular laws should protect the fundamental rights - life, liberty, and pursuit of happiness - of members of society from the actions of other members of society.

Religious laws should show a person how to maintain a proper relationship with God. The ultimate punishment that religious laws should be able to impose is expulsion from the community of faith - if you won't obey the laws of the (for example) Muslim faith, then you are not a Muslim. If you refuse to accept the Nicene Creed, you are not a Catholic. No physical penalties, just spiritual ones. If the faith is a sham, the spiritual penalties don't matter (a nonexistent deity couldn't condemn anyone to hell); if the faith is real and true, then the spiritual penalties are far worse than any civil penalties could ever be.

In some cases, secular and religious laws may coincide - "Thou shalt not bear false witness against thy neighbor" is one example - but the underlying premises and consequences are different.

Now, there are cases where one person may believe that a law is purely religious in nature, while another believes it is also by nature secular, but that's another dispute altogether.