Q. How do you deal with a sinning professed believer who is not a church member?
This is a good question. I find it interesting to observe (over a good number of years) that while churches are all too often slow to deal with sin in the church, sinning church members are often much quicker to leave the church on their own (before discipline can occur).
Church discipline (in its most drastic form -- expulsion from the church) removes the sinner (who claims to be a saint -- 1 Corinthians 5:9-13) from the church, and thus from its benefits and protections. A person who claims to be a Christian, and yet does not fellowship in and with a church is sinning (Hebrews 10:25). In effect, the person who withdraws from the church (or refuses to be a part of a church) has already placed themselves under God’s discipline.
Having said this, I note from 1 Corinthians chapter 5 that Paul personally exercises church discipline from afar (1 Corinthians 5:3), serving as an example to the church.
I believe it is clear in Scripture that church discipline requires both individual and corporate (the entire church) action. For example, Matthew 18:15-20 instructs the individual who becomes aware of the sin of another to confront the sinner with their sin. If repentance does not result, 2 or 3 others should be included in the rebuke. And ultimately, if repentance is refused, the entire church must become involved. Once discipline is exercised by the church, every member of the church is required to follow through with the action of the church. Eating and having fellowship with such a person on a personal level should cease, in my opinion, but always with the hope of repentance and restoration. That does not mean there can be absolutely no contact with this person, but it would be greatly restricted (Matthew 18:17).
In the case which you have presented, I believe that discipline should be exercised on a personal level, and that he or she should first be confronted regarding their sin, and then should be dealt with as Matthew 18:15-20 and 1 Corinthians 5 indicate, on an individual basis.
While a willful sinner may attempt to give the impression that a rebuke has no impact on them, it is, I think, worthwhile to warn them that in Paul’s words, church discipline is “turning the wayward sinner over to Satan for the destruction of the flesh” -- 1 Corinthians 5:5. This means that bad things may result. This might get their attention, and may very well come to mind if and when their life begins to fall apart.
I remember an occasion when I was involved in the discipline of an individual who was not part of our church. As the ambulance was hauling this fellow off after a heart attack, he yelled out, “And tell ____________ that this is not God’s discipline!”
I hope this helps,