"Has God rejected them forever because they sinned?". Paul uses this question to introduce the next thing he wants to say. If your language uses a different way to introduce new topics, you can use it here. Here the words "stumble" and "fall" refer to sinning and having God reject the sinner.
"That is not possible!" or "Certainly not!" This expression strongly denies that this could happen. You may have a similar expression in your language that you could use here. See how you translated this in Romans 9:14.