Many Protestants teach that you cannot lose your salvation, so when a person "falls away" from the faith, some of these Protestants conclude that this person was never really saved in the first place. Their favorite prooftext for this claim is 1 John 2:19. Their interpretation is quite convenient, but is actually quite unreasonable, and it's is hurtful towards Christians who struggle with sin (thinking they might never have been saved).
To begin, consider the context of 1 John 2:19,
18 Children, it is the last hour, and as you have heard that antichrist is coming, so now many antichrists have come. 19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us.
These Protestants read 2:19 as saying these people who "went out" of the community by apostasy demonstrate that they never really were saved, since true Christians remain in the community. This interpretation is somewhat understandable, but it is very weak when you consider the context, the Greek words themselves, similar verses, and theological coherence.