When people abandon Christ, they prove that they never had faith at all, since those with true faith are preserved by God, never to fall away permanently (1 John 2:19). Still, when we see people leave Jesus, it does not necessarily mean there is no hope for them. Only God knows the state of a person’s heart, and He may be pleased to bring one who has backslidden back into His fold. That is why we must never cease praying for those who have left the church.
For further study:
The Bible in a year: