Charles Spurgeon Commentary 1 Thessalonians 1:5

"how that our gospel came not unto you in word only, but also in power, and in the Holy Spirit, and [in] much assurance; even as ye know what manner of men we showed ourselves toward you for your sake." — 1 Thessalonians 1:5 (ASV)

Paul never had a happier time in preaching, it would seem, than when he preached to these Thessalonians. He felt a power resting upon him. He spoke the gospel with great positiveness and assurance, and consequently the people received it in power, and the assurance of the hearer made the assurance of the speaker. It is a great mercy when it is so.