Questions: Paul mentions in 1 Corinthians 5 that the fornicating man is to be handed over to Satan for the destruction of the flesh so that his spirit may be saved in the day of the Lord. How can his spirit be saved in the day of the Lord if he is not a Christian?
Answer: I would see the “wicked person” (1Cor 5:13) who had his father’s wife (1Cor 5:1-5) as a child of the devil (1Joh 3:8). The congregation was instructed to “put away from among yourselves” (1Cor 5:13) (excommunicate) this person.
The object of putting this person away was not only for the protection of the congregation (1Cor 5:6) but so that this person would be “saved in the day of the Lord Jesus” (1Cor 5:5).
Not leaving him in the congregation with a false profession of godliness (2Tim 3:5) and no longer fellowhsipping him but treating him as a “heathen man and a publican” (Matt 18:17) was to have the effect of “deliver such an one unto Satan for the destruction of the flesh” (1Cor 5:5).
I would understand this to mean that with him no longer being accepted as a Christian, allowed to attend meetings, or fellowshipped by the saints he would be inclined to drop his false profession of salvation and plunge into deeper sin. The hoping being that his sins would cause him to “came [come] to himself” and realize that he was “perish[ing] with hunger” and that at the father’s house there was “bread enough and to spare” (Luk 15:17).
It would be the decision to “arise and go to my father, and will say unto him, Father, I have sinned against heaven, and before thee” that would cause him to be “saved in the day of the Lord Jesus” (1Cor 5:5).