5 For our gospel came not unto you in word only, but also in power, and in the Holy Ghost, and in much assurance; as ye know what manner of men we were among you for your sake.
In order to indicate that he showed all humility among the Thessalonians and in order to adopt a gentle approach in dealing with them, Paul makes it clear that none of the powerful means by which they have been brought to faith is lacking. His preaching was accompanied by signs and wonders and was empowered by the Holy Spirit. It embodied the fullness of truth rather than fantasy. This could be seen in the teaching of grace, in the quality of discourse among them and in the healing of the sick. By these they could be seen to be the true heirs of the gift of God in the promises to Abraham.
Thus, the obedient and responsive soul gives itself over to the virtuous life. This life is freedom itself, on the one hand, from the chains of this life, separating itself from the slavery of base and empty pursuits. On the other hand, this soul devotes itself to faith and the life of God alone, because it sees clearly that where there is faith, reverence and a blameless life, there is present the power of Christ, there is flight from all evil and from death which robs us of life. For shameful things do not have in themselves sufficient power to compete with the power of the Lord. It is their nature to develop from disobedience to his commands. This was experienced in ancient times by the first man, but now it is experienced by all of us when we imitate Adam’s disobedience through stubborn choice. However, those who approach the Spirit with honest intent, unfeigned faith and an undefiled conscience, are cleansed by the Spirit according to the one who says, “for our gospel was not delivered to you in word only, but in power also; and in the Holy Spirit and in much fullness, as you know.”
"Knowing, brethren beloved of God, your election, how that our Gospel came not unto you in word only, but also in power, and in the Holy Ghost, and in much assurance; even as ye know what manner of men we showed ourselves among you for your sake."
Knowing what? How "we showed ourselves among you"? Here he also touches upon his own good actions, but covertly. For he wishes first to enlarge upon their praises, and what he says is something of this sort. I knew that you were men of great and noble sort, that you were of the Elect. For this reason we also endure all things for your sake. For this, "what manner of men we showed ourselves among you," is the expression of one showing that with much zeal and much vehemence we were ready to give up our lives for your sake; and for this thanks are due not to us, but to you, because ye were elect. On this account also he says elsewhere, "And these things I endure for the Elect's sake." (2 Tim. ii. 10) For what would not one endure for the sake of God's beloved ones? And having spoken of his own part, he all but says, For if you were both beloved and elect, we suffer all things with reason. For not only did his praise of them confirm them, but his reminding them that they too themselves had displayed a fortitude corresponding to their zeal.
The reception of the gospel by the Thessalonians “in power” may mean that Paul’s preaching was accompanied by miraculous signs, but it may also mean that it strengthened the Thessalonians for much endurance in suffering.
For you know, he says, how you came to be chosen (that is, how you arrived at faith), for you did not believe solely by means of our words. Indeed, we not only spoke but also showed wonders, great and glorious and worked by the Spirit, by which you were strengthened with regard to the things we had spoken to you. Hold on tightly to what you have learned, and don’t be deflected from your course by the external things that happen to you.
From this, he says, your election is evident, that God glorified the preaching among you. For we did not simply preach, but there were also signs, because God was pleased that you should believe as chosen by Him and foreordained.
That is, in signs that served for the benefit of well-minded people and for the punishment of ill-minded ones.
Or in spiritual wisdom, and not worldly; or in that the Holy Spirit was given to those who believed.
That is, in afflictions and sufferings. Just as the sign and gift of the Holy Spirit served for the full assurance of those who believed, so also the sufferings endured for the sake of the preaching constitute a great confirmation of it.
You, he says, are witnesses of how we conducted ourselves among you and with what great readiness we preached; but this too is for your sake. Since you are chosen, we labored so zealously for you. For what would one not endure for the beloved of God? Here he also touches upon his own struggles, though in a veiled manner, since he wishes first to praise them.
What is said in Ezekiel (3:26) would seem to contradict this: “And I will make your tongue cleave to the roof of your mouth, so that you shall be dumb.” To counter this Paul first calls to mind how powerfully he preached to them; secondly, he calls upon their own witness with the words: “you know...” Powerfully, because he came not in loftiness of speech, but in power: “And my speech and my message were not in plausible words of wisdom, but in demonstration of the Spirit and power” (1 Cor. 2:4). “For the kingdom of God does not consist in talk but in power” (1 Cor. 4:20). Now this may have reference either to the authentication of his preaching or to the manner of his preaching. If it is the first alternative, then Paul's preaching to them was authenticated not by arguments but by the power of signs, and so it is said in Mark (16:20): “The Lord worked with them and confirmed the message by the signs that attended it”; and by the giving of the Holy Spirit; so Paul says, “and in the Holy Spirit.” “While Peter was still saying this, the Holy Spirit fell on all who heard the word” (Acts 10:44). “While God also bore witness by signs and wonders and various miracles and by gifts of the Holy Spirit” (Heb. 2:4). “And with full conviction.” Paul adds this so that they would not believe that they received less than the Jews, indicating that the Holy Spirit does not discriminate among persons; but that the preaching was in the same fulness among them as among the Jews: “And they were all filled with the Holy Spirit” (Acts 2:4).
But if it is the second alternative, then “in power” seems to mean “showing you a virtuous life.” “Jesus began to do and teach” (Acts 1:1). “And in the Holy Spirit” who brings things to mind; “For it is not you who speak but the Spirit of your Father speaking through you” (Matt. 10:20). “With full conviction,” because I have instructed you in everything necessary for the faith. And he appeals to their testimony on this point when he says: “You know what kind of men we proved to be among you for your sake,” that is, what kind of gifts and powers we have exhibited among you: “I hope it is known also to your conscience” (2 Cor. 5:11).
[AD 384] Ambrosiaster on 1 Thessalonians 1:5