2 Corinthians 2:9

For to this end also did I write, that I might know the proof of you, whether ye be obedient in all things.

The letter was written to this end, to learn by proof whether they would be obedient in all things. The obedience under test in that situation included receiving the repentant sinner again, welcoming him back with love and kindness through forgiving him and restoring him. The forgiving decision the church made toward him would be confirmed and upheld as done in Christ's stead, with the authority of His representative.