Charles Ellicott Commentary 2 Thessalonians 1:6

"if so be that it is righteous thing with God to recompense affliction to them that afflict you," — 2 Thessalonians 1:6 (ASV)

Seeing it is — Literally, if it is indeed fair. This is a form of speech common to Paul, used when he argues from a fact he knows his readers will recognize (for example, Romans 8:9). The argument is: “Your persecution is a clear indication of what God’s righteous verdict will be—that He will pronounce you worthy—unless you deny (which you will not) that it is fair to repay the persecutors with tribulation and the persecuted with rest.” The context shows that Paul does not mean that all suffering deserves to be rewarded with bliss, but he does present it as a matter of common fairness that when people have suffered for the kingdom’s sake, God should reward them in the life to come.

With God — Such a system of repayment seems fair to humanity; is it likely to seem less fair in the eyes of God? Holy Scripture consistently declares the power of the human conscience to recognize God’s principles of action: whatever is righteous for people is also righteous for God, and vice versa.