John Calvin Commentary 1 Thessalonians 2:10

"Ye are witnesses, and God [also], how holily and righteously and unblameably we behaved ourselves toward you that believe:" — 1 Thessalonians 2:10 (ASV)

You are witnesses. He again calls God and them to witness, in order to affirm his integrity. He cites God, on the one hand, as a witness of his conscience, and them, on the other hand, as witnesses of what they had known by experience. How holily, he says, and justly, that is, with how sincere a fear of God, and with what fidelity and blamelessness towards men; and thirdly, unreproachably, by which he means that he had given no occasion for complaint or reproach.

For the servants of Christ cannot avoid slanders and unfavorable reports; because they are hated by the world, they will inevitably be spoken evil of among the wicked. Therefore, he restricts this to believers, who judge uprightly and sincerely, and do not revile maliciously and groundlessly.