1 Thessalonians 3:3 KJV
That no man should be moved by these afflictions: for yourselves know that we are appointed thereunto.
The appointment of which Paul speaks might be either "appointed to afflictions" and it is true that not only the Apostle, but indeed all believers could consider themselves appointed to fill up the afflictions of Christ. Or else it could refer to the work he had sent Timothy to do among them. He said Timothy was his fellow laborer in the Gospel and had been sent to establish them and to comfort them in the faith so that they would not be moved by the afflictions they suffered.
God appoints certain ones; like Paul, like Timothy and like pastors today to
Labor in the Gospel
This would be first and foremost the work of evangelization or persuading men and women to come to Jesus Christ as redeemer of their souls.
Establish the believers
Once a person is saved our work has only just begun. We are to establish them in the faith. This is the teaching ministry where the preacher carefully and persistently shows them the truths that the Bible gives us and how they apply to personal life.
Offer to them comfort when they struggle for their faith.
All who live godly in Christ Jesus shall suffer persecution. God has appointed us to the task of being a comfort to those who are in any way afflicted. We do that through the comfort wherewith we ourselves have also been comforted of the Holy Ghost.