Some things you could try if you are still in dialogue, are some "diagnostic questions."
"Do you understand that God intended us to keep the Sabbath?"
"what would it take to convince you of the sabbath?"
"what would it take for you to keep the sabbath?"
These help to know where they are at.
sometimes they will just come right out and tell you that they would keep it ...but ....and then you get their reason. Sometimes it is lifestyle. Sometimes they are not convinced. You have to look at what level they are on.
A good book on the questions is by Mark Finley entitled "Persuasion". It used to be called something else, and I really don't like the new title. It implies that we are used car salesmen or something. But the information is good.
People make decisions on 3 levels.
a. information
b. conviction
c. motivation.
Your first goal is to see which of these is the hang up.
Someone might be convinced of the facts, but not convicted that they are to do anything. Or someone might be confused on the facts...no amount of appeals will help till you back up and inform some more.
Some are under conviction, but don't feel any motivation, because it goes against their lifestyle. The key is to find out where they are.
Keep in mind, this is not just a cerebral process. Pray that God would direct you in your dealings with them.
Finally, when they become contentious, I would step back a bit and say
"Are you sincerely looking at the facts, or is your mind made up? "
If they say their mind is made up, then agree to disagree and move on.
I have made the mistake before of doing the hard sale of beating the Sabbath into a person. It does nothing but make them resent you. If God does not bring conviction and decision, then you can't either. Give the information, give clarification, give scriptures that help to clarify conflicts in motivation, then let it go.
A friend of mine, also a pastor studied with a man once who was convicted on the Sabbath but had no desire at all to keep it. The man straight out said:
"I was born a Lutheran, I am going to die a Lutheran, and I am going to hell a Lutheran."
I think the pastor got the point. "Get out, stop talking about it, my mind is made up. "
Once that happens, or usually even before, to argue is pointless. You have to listen to how much people can move at ONE TIME. Let them off the hook for a bit. THey will think it over. They don't have to respond to your appeal immediately. Even the White's did not accept Bates presentation the first time. God can deal with them.
On a related note, an interesting story was told by Phillip Samaan about this process. He was doing one of his first Bible studies, and was convinced that if he could get the young man to accept the Sabbath everything would fall into line.
After 4 studies on the subject the young man came out and said "I can't stand to spend 5 minutes with God, and you want me to spend a whole day with him."
I think his answer is helpful. If people are not interested in being with Jesus, or if we don't relate well enough how that is a BLESSING rather than a curse, then no amount of arguing will help your cause. Make sure you begin by looking at salvation, then go to issues such as the Sabbath. The Sabbath is a great message that we have a day to spend with the One who bought our salvation. If viewed as just a requirement that causes them to lose friends and be miserable, they will never decide for it.
As for character assasination. That is a hard one. Jesus says that they will treat us this way. The sooner you can simply relate that they have their choices to make and you have yours, the sooner you can get on with life. Sometimes we WON'T agree. So don't feel you have to keep pressing the issue with them if they are not willing to listen. If we constantly push someone who does not want to give in we make them fight out of the corner even more fiercely. Don't push them into a position they have to defend. Show them the message, clarify it, then let them think. Conviction will do its work, or they will reject it.
