Romanos 11:21

Throughout this whole discussion Paul was viewing Gentile believers and Jewish unbelievers as two groups. This fact is clear from his use of the singular "you" in the Greek text (su, Romans 11:17-24). If he had been speaking of individual believers, we might conclude that this verse provides some basis for believing that a believer can lose his salvation. Paul’s point was, if God set aside Jews temporarily because of their unbelief, He could do the same with Gentiles because of their boasting.