This full conviction
is a profound assurance that certain beliefs are true, and refers to the Thessalonians’ deep inward persuasion of the truth of the gospel, a token of the Holy Spirit’s work in their hearts, more impressive and more lasting than the persuasion produced by spectacular or miraculous signs.
1 Since the gospel message was diametrically opposed to everything that the Thessalonians had previously believed, the fact that they came to trust in that message and in the Saviour whom it proclaimed was a powerful indication that God himself had been at work in their hearts.
5 because our gospel came to you not only in word, but also in power and in the Holy Spirit and with full conviction. You know what kind of men we proved to be among you for your sake.