Many Protestants believe that once one is saved by Christ then they are not able to fall away from grace and lose their salvation. In other words, they believe that if a person has truly been saved by Christ they will never choose to fall away from the faith.
Much can be said about this doctrine from a Catholic perspective and many Scritpures can be used to refute this view, but one that has caught my attention in the last few years is from the Book of Galatians 3:27 and Galatians 5:4.
Galatians 3:27 says that those who have been baptized have “put on Christ”. Clearly, Paul is talking to people who have been saved. Yet later on in Galatians 5:4 he rebukes these same people for trying to be justified by the works of the Law of Moses (in other words, saying that one has to become a Jew first before they can be saved). He tells them that if they try to be justified by the Law (of Moses) then they have fallen “away from grace” and have been “severed from Christ”. If then it is not possible to forfeit one’s salvation, then how can Paul say that the Galatians have put on Christ and later tell them that they have been severed from Christ? It is not as if they were merely affiliated with Christ by a false profession since Paul tells those that have put on Christ in 3:29 that “if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.” Clearly these are people who have been united to Christ and have become children of Abraham, having been justified as their forefather Abraham was justified. And yet Paul tells the very people who have put on Christ and become offspring of Abraham that those who have attempted to be justified by the Law of Moses have been severed from Christ.
In light of these passages among others, as well as the tradition of the earliest Christians going back to the Apostles, Catholics teach that one can truly be saved and yet choose to forfeit their salvation if they so choose. It is a scary thing but God does not take away our will after salvation, if we are so inclined we can foolishly choose to reject Him.