RC Sproul attributes it to Romans 9:
The combination was too much for me. Gerstner, Edwards, the New Testament professor, and above all the Apostle Paul, were too formidable a team for me to withstand. The ninth chapter of Romans was the clincher. I simply could find no way to avoid the Apostle’s teaching in that chapter. Reluctantly, I sighed and surrendered, but with my head, not my heart. “OK, I believe this stuff, but I don’t have to like it!”
God is Open 
I read a book by R.C. Sproul about 35 years ago – before I knew the difference between a Calvinist and a Arminian. My opinion of him then was that he was confused. I have not changed my mind. Calvinism and Arminianism are both wrong. The truth is Universalism.
He is claiming Paul would be a Calvinist? Really? How can one read Paul and come to that conclusion?
Exactly, it is almost as if Sproul completely missed Paul’s actual points and in spite of authors like NT Wright, he refuses to entertain better readings of Romans 9.