Having said "not in word only," he clearly wishes next to say something besides word. Therefore the words "in power and in the Holy Spirit" should not be understood as qualities of the word itself.
They should be understood as something besides the word, though accompanying the word. The living word is a necessary means for spreading the Gospel, but by itself it is insufficient.
A special assistance of God gave it power, as the Evangelist Mark testified concerning the Apostles. They went out and preached the Gospel everywhere, while the Lord worked with them and confirmed their word by the signs that followed (Mk 16:20).
This same thing Saint Paul says here. Their gospel was not in word only, but in power and in the Holy Spirit. "In power" means in signs manifested in Thessalonica by God through Saint Paul.
"In the Holy Spirit" means in the sending down of wondrous gifts of the Spirit, such as the gift of tongues, prophecy, and the others so common in the first Christian Churches. He hints at this when he says at the end: "Do not quench the Spirit. Do not despise prophecies" (1 Thess 5:19-20).
Blessed Theodoret says on this passage: "You yourselves are witnesses of the power of the Gospel. For we not only offered you an instructive word, but also proved the truth of the words by miracles.
"We also communicated to you the grace of which we ourselves are partakers. This is what the words 'and in the Holy Spirit' mean."
Saint Theophylact adds: "From this it is clear that you are elect, because God glorified the preaching among you. For we did not simply preach in that way, but signs also took place, because God blessed you to believe as those elected and foreordained by Him."
By the words "in much assurance," Saint Paul means that their preaching, accompanied by miracles and the distribution of the gifts of the Holy Spirit, penetrated the hearts of the Thessalonians.
It produced in them full and perfect conviction, or poured into their hearts the riches of the full assurance of understanding (Col 2:2). The Savior also said that without signs and wonders one cannot expect faith from men (Jn 4:48).
The tangible presence of God's power softens the heart, opens it, and lays it open to receive the truth. But in these words one may see an indication not only of conviction, but also of some special manner of receiving it.
Besides the signs proceeding from the Apostle and the gifts of the Holy Spirit distributed by the laying on of his hands, various persons could, by God's mercy, receive personal assurances and confirmations of the truth of the Gospel.
One might receive a dream, another a vision, another inward illumination. In a similar way, Saint Paul himself was specially informed not to go into Asia, but to Macedonia.
The comparison of these words with "in power and in the Holy Spirit" compels us to suppose this. Just as the right hand of the Most High is evident in those words, so nothing prevents us from seeing its indication also in "much assurance."
The next verse speaks of the conviction and faith of the Thessalonians. There is nothing surprising in the fact that such features accompanied the preaching of the Gospel in Thessalonica. This was to be expected.
The Spirit did not permit the holy Apostle to go into Asia and Bithynia, and the man who appeared called him to Macedonia. This indicates both that the time of calling had arrived for the Macedonians and that God's assisting hand would not be shortened.
How natural it was that when Saint Paul, guided by this invisible man and by the Spirit, reached Thessalonica after the miracles in Philippi and extended to them the word of the Gospel, this word was confirmed by signs and assurances from above.
"As you know what kind of men we were among you for your sake." Since the Apostle's aim is to awaken in the Thessalonians conviction in the divinity of the holy faith they received, he first indicates the ground of his own conviction in their election.
Saint Paul now brings the same facts into the consciousness of the Thessalonians. By considering all that happened among them during the preaching, they too should draw the same conclusion.
They should say: yes, here the finger of God was tangibly evident, acting among us. God's great mercy toward us, marking election, is beyond doubt.
Saint Paul does not say how we preached among you, but "what kind of men we were," as if to say: you know what signs God showed among you through us.
You know what gifts you received, and how God assured you that our word is true. Therefore do not doubt God's good pleasure toward you or the divinity of the faith.
"What kind of men we were among you for your sake." Everything that came from us was for your sake. We are instruments; the source is God; you are the receivers.
Passing beyond the instruments, look to the source, and be assured from everything that "the kingdom of God has come upon you" (Lk 11:20). It came according to God's will, according to His love for you and His election of you.
From this, what the Apostle was leading them to followed of itself: therefore stand in the faith, counting all deprivations and attacks as nothing.