It's not possible to have a rational argument with someone who actually believes in something? With someone who believes that objective moral truth exists and can be known with reasonable certainty?
I find your seeming willingness to subordinate conscience to the state, and to require others to do so, troubling. A good state does not require persons with a rightly formed conscience to do that, simply because a good state does not pass laws that contradict natural law. There aren't many good states today.
I do not require that one subordinate one's conscience to the state, rather that one should take an oath to uphold the laws of the state, and then claim the right to violate those laws on account of religious scruples. Or, in the case of the duties of all citizens, one must accept the secular consequences of placing religious conscience ahead of those duties. That was the idea of civil disobedience: that if you break the law, you accept the punishment. After all, it's a cheap conscience that is not willing to accept adversity as the price of following it.
The further difficulty with your argument, and indeed with trying to have a discussion with you, is that people of good conscience may well, and do, disagree on what constitutes natural law or the truth. To argue that any particular version of truth is subject to debate is not to argue that there is no absolute truth, rather that we know it only imperfectly.