Here’s Matthew 24:13:
“But the one who stands firm to the end will be saved.”
In context Jesus is talking about the great end-times troubles that will engulf the world in the last days in which believers will face enormous spiritual opposition. Those who stand firm in their faith will be delivered. Those who renounce Christ in that day will be revealed as never having had saving faith at all. This isn’t talking about a momentarily lapse as much as a calculated decision to reject Christ and all his benefits and to publicly renounce him. A person who does that could hardly have been a believer in the first place.
If, finally, when the time for decision comes, a person says, “No, I renounce Jesus and declare that I do not believe in him,” it is hard to see why we should regard that person as ever having been a born again Christian. The pressure of those days will reveal the truth about every heart. Some will be revealed as a “sunshine believers” whose faith withers under persecution.