Most problems people experience when arguing with religious people is they don't put forward their case correctly. They treat the dispute like any other, and fail to adapt their argument to the actual situation.
In an ordinary argument: Person A says murder/theft/adultery is wrong. Person B asks why. Person A replies that Person C says it’s wrong. Person B can then claim person C is either not in a position to know, or is lying, or for another reason, is plain wrong.
Now, replace person C with God, person B with Lagrange, and Person A with SFC. Obviously, Lagrange is not going to get anywhere arguing God is not in a position to know, or is lying. What he should be concentrating on instead is the transmission mechanism between God and SFC. So it could go something like this:
Lagrange: How did God tell you it is wrong? Did he tell you Himself?
SFC: No, He told me in the Bible.
Lagrange: Did God write the bible personally?
SFC: The bible is the Word of God, was inspired by God, and so represents absolute truth.
Lagrange: But did God read you the Bible?
SFC: No. I read it myself.
Lagrange: Did God tell you what it means?
SFC: No, my priest/ preacher told me.
Lagrange: Are you and they infallible? Does the Word of God make sure all Christians always understand the same thing when they read the Bible?
SFC: Evidently not.
This gives room for further argument, which might even prove constructive. Everyone holds something in their life to be special, and feel sensitive when people criticise it. So don’t run around trying to criticise God. It is not God who you have a problem with. It is people who are stubborn. Believe it or not, when you get to know people, they may have more depth to them then you at first assume, and can justify their beliefs in ways which don’t require a narrow scriptural understanding.
Do Say: “I believe you are taking that biblical saying out of its true context. Jesus was actually talking about…” or “I disagree with your interpretation of the parable in question, we are meant to understand from it that…”
Don’t Say: “God said this!” “No, God said this!” or “You’re an infidel!” “No, you’re, an infidel!”