One of the terms often used of believers who have fallen away from Christ is backslider. Though the word backslider is not found in the New Testament, the concept is still taught. A backslider is a Christian, who has fallen into sin and out of fellowship with God. It is important to note that the backslider is still a Christian. It is not a believer who has lost his salvation. Salvation is a free gift from God and cannot be lost once the gift has been received.  

Loss

His condition is similar to the prodigal son (Luke 15). He has fallen out of fellowship with God and gone to the world to find fulfillment. Any pleasures enjoyed are only temporary. When a person backslides or falls away from the Lord they will lose their fellowship with God, not their salvation.

Corinthian Church

The New Testament provides us with an account of the backsliding of some members of the Corinthian church. Their sins were of such a nature that God sent sickness and death to some of them. They were backslidden and had died while in that state. And yet they went to heaven:

For this reason many are weak and sick among you, and many sleep. For if we would judge ourselves, we would not be judged. But when we are judged, we are chastened by the Lord, that we may not be condemned with the world  (1 Corinthians 11:30-32).

Disciplined

When believers backslide, they are then disciplined by the Lord. Those whom the Lord loves He disciplines:

For whom the Lord loves He chastens, and scourges every child who He receives. If you endure chastening, God deals with you as children; for what child is there whom a father does not chasten? But if you are without chastening, of which all have become partakers, then you are illegitimate and not children (Hebrews 12:6-8).

Summary

A backslider is a believer who has fallen out of fellowship with God. Though a biblical term, the word backslider is not found in the New Testament. Those believers who have fallen into sin will be chastised by Him.