I'm not sure where this is, perhaps Romans or Corinthians, but it goes something like this; "we are not saved by sabaths, holy days, or festivals of the moon, we are saved by faith, and that in Jesus Christ." I'm sure I have it wrong. Perhaps someone can correct me here? All the same, I find using that one will work for the new Christian who is enamoured with the Law.
Is your friend an SDA? If so, you have no hope in this argument. They are enslaved with the Law and refuse to listen to what Scripture says about that. If he is Lutheran, then I would suggest discussing with his source that what they are telling him is Heterodox. In fact, I would suggest having the Pastor speak to both parties. You just might have a wolf in your sheepfold.
Luther, I love the Lincoln quote. What is the context of it, wherever did you find it? It often amazes me that Academia insists that so many of our Presidents weren't Christians. Yet, if one studies their collected works, that's exposed as a lie. Then again, where is that a surprise?