He went on his way through towns and villages, teaching and journeying toward Jerusalem. And someone said to him, “Lord, will those who are saved be few?” And he said to them, “Strive to enter through the narrow door. For many, I tell you, will seek to enter and will not be able. When once the master of the house has risen and shut the door, and you begin to stand outside and to knock at the door, saying, ‘Lord, open to us,’ then he will answer you, ‘I do not know where you come from.’ Then you will begin to say, ‘We ate and drank in your presence, and you taught in our streets.’ But he will say, ‘I tell you, I do not know where you come from. Depart from me, all you workers of evil!’ In that place there will be weeping and gnashing of teeth, when you see Abraham and Isaac and Jacob and all the prophets in the kingdom of God but you yourselves cast out. And people will come from east and west, and from north and south, and recline at table in the kingdom of God. And behold, some are last who will be first, and some are first who will be last.” (Luke 13:22-30 ESV)
There are many who think they are Christians—just take a look at current polls of Americans considering themselves to be saved or biblically minded. However, Scripture is clear that while many think that they’re a part of the true Church, in fact very few will be saved. They are many who are self-deluded and deceived. As Daniel Dyke wrote, “Oh how many sleepers are there, that dream this dream of a strong assurance of their salvation, that both live and die in this dream, and so go down merrily into hell!” The relevant question here really isn't "how many will be saved" but "have you been delivered from your sin and God's pending judgment through faith in His Son, Jesus Christ?"
So how is the Church to handle this? I believe the proper place to start is rather than speaking to every Sunday morning churchgoer as a child of God, pastors / elders, teachers, family members, friends need to warn, persuade and when necessary plead that everyone examine his or her heart by the light of God’s Word. The purpose here isn’t to create perpetual doubt. Instead, since the danger is very real and prevalent among so many, we are to be jolted out of any wrong assumptions. This is serious stuff and we have a conniving enemy who not only masquerades as an angel of light, but also dresses false conversions up as genuine faith.