Children, it is the last hour, and as you have heard that antichrist is coming, so now many antichrists have come. Therefore we know that it is the last hour. 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 (1 John 2:18-19).
I have known Christians that simple have no assurance. They grew up listening to sermons that would make them feel like God will take away their salvation as if it were their first car after getting their driver's license. There are a few denominations that teach believers can lose their salvation. Yet, there are people in the church that say they are a Christian, but live like they are on their way to hell. There are also believers that have simply walked away from the Christian faith for various reasons.
John Piper in this video lab answers the question, can a true Christian fall away from God? Can a true believer lose their salvation?