1. I would hazard an educated guess that R.C. Sproul was taught this idea re: Luther's writings on predestination, by Dr. John Gerstner. This is NOT to suggest that R.C. Sproul didn't do his own research and come to the same conclusion.

2. Luther did write on the subject of predestination in his Bondage of the Will, basing his view, at least in part, on Ephesians 2:8–10.

3. see here: Double Or Nothing: Martin Luther's Doctrine of Predestination by Brian G. Mattson (1997).