Part 3 of Cranfield looking at 'mercy' in his intro to Rom. 9

"(iii) It is only where the Church persists in refusing to learn this message, where it secretly - perhaps quite unconsciously! - believes that its own existance is based on human achievement, and so it fails to understand God's mercy to itself, that it is unable to believe in God's mercy for still unbelieving Israel, and so entertains the ugly and unscriptual notion that God has cast off His people Israel and simply replaced it by the Christian Church."

[Cranfield, Romans]