I was asked about church discipline for a young adult who has made some very bad moral and spiritual choices. The question was, When does 1 Corinthians 5 come into play? That answer is, it comes into play at the end of the process, not at the beginning. Evidently the sordid situation inside the church at Corinth had been festering for a long time and the church had even boasted about it ("Look how tolerant we are!"). In that case the offender must be publicly delivered over to Satan (cast out into the world) not only in judgment but in prospect of his eventually restoration.
The whole process Jesus laid out in Matthew 18:15-17 is exactly that–a process. You don’t start by “telling it to the church” (see v. 17). You take that step only as a last resort when repeated appeals for repentance have been rejected.
As we deal with those who have been ensnared by sin, we need wisdom and we need patience. In our grief, frustration and anger over what has happened, we may act hastily or harshly. If someone says, “I am leaving the Christian faith,” it may not be their final word. We all say things that we regret later.
Give God time to work.
I discuss this in more detail in How to Handle Trouble in the Church. In the meantime don’t start with the final step. There is a time and a place to go public, but that’s at the end, not at the beginning.