2 Thessalonians 1:10
…when He comes to be glorified in His saints on that day, and to be marveled at among all who have believed — for our testimony to you was believed.
I’m a little confused by the logic here. Paul is saying those who believed his testimony will marvel…because they believe? What does he mean by the last phrase: “for our testimony to you was believed.”?
Paul was simply saying that "When the Lord Jesus shall be revealed from heaven, with his mighty angels, in flaming fire taking vengeance on them that know not God and that obey not the gospel of our Lord Jesus Christ" - then the Christians alive at that time will marvel at seeing Christ (verses 7 to 8).
In verses 4 to 7 Paul knew about the tribulations and persecutions those Christians in Thessalonica had already endured. He said it was righteous for God to recompense tribulation to those who troubled them. That would happen at Christ's glorious return, with the angels, for the Day of Judgment and Resurrection. All the Christians in the first century expected that could happen at any time. They did not know when Christ would return, but they had total faith in his promise that he would.
You ask explicitly what the last phrase means: “for our testimony to you was believed.”? It was the testimony of Paul and the Apostles that the crucified Christ was raised from the dead, ascending to Heaven, from when he would return one day for that spectacular Day of Judgment and Resurrection. They believed that they would be caught up to join Christ in the sky, where the dead in Christ had just gone.
Paul had earlier written to them about that, in his first letter to them. Read that in 1 Thess. 4:13 to 5:6. In part, Paul said:
"For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him... we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep. For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air, and so shall we ever be with the Lord." - A.V.
For nearly 2,000 years now, that glorious Day of wonder and admiration awaits. We still do not know when it will suddenly happen but we are to be prepared with the same strong faith and expectation that those first century Thessalonians had. Our wonder and admiration in the appearing of our Saviour with hosts of angels will know no bounds, whether we are still alive on Earth when it happens, or whether we will have died by then and so have the honour of being with Christ at his return, clothed with resurrected bodies.
This has to be believed by all who have faith in the return of Christ. We today who are Christians believe the same testimony of Paul and the Apostles, otherwise we cannot call ourselves Christians, nor will we be thrilled at Christ's coming for and with the saints - those who do not believe that testimony will be terrified, as Revelation 1:7 says.
It is not necessary to wander into interpretations of end times to answer this question, for you simply ask what the logic is in that one verse. The logic becomes apparent when we read what Paul had previously written to them on that subject, and tie it in with his further information about how amazed they will be to see Christ and the angels, not to mention Christians who had died previously.
It is possible that in 2 Thess 1:10 Paul is alluding to the famous verse on Isa 25:9 -
And in that day it will be said, “Surely this is our God; we have waited for Him, and He has saved us. This is the LORD for whom we have waited. Let us rejoice and be glad in His salvation.”
This is in contra-distinction to the wicked at the same event of the Lord's return who have the opposite reaction as recorded in Rev 6:15-16 -
Then the kings of the earth, the nobles, the commanders, the rich, the mighty, and every slave and free man hid in the caves and among the rocks of the mountains. And they said to the mountains and the rocks, “Fall on us and hide us from the face of the One seated on the throne, and from the wrath of the Lamb. (See Hos 10:8, Luke 23:30)
In 2 Thess 1:10, it is only the believers who are glad to see the Lord return and marvel at His glory.
When Jesus returns, all the believers are going to be transformed to His likeness, to truly be sons and daughters of God. That is the promise. In which God will be glorified in similar way that a masterpiece glorifies the artist. At the same time, all the believers will marvel at God's goodness and generosity.
And since the Thessalonians believed the good news that Paul shared with them, they will be part of that reality. They will be among all the believers too.
I suggest the last clause is almost an explanatory aside. Why does the category of "all who have believed" exist? It exists because "our testimony was believed". They believe as a result of the testimony.
When Christ returns he will he will stop the Tribulation that is about to destroy everything:
And unless those days were shortened, no flesh would be saved; but for the elect's sake those days will be shortened.
— Matthew 24:22 (NKJV)
Paul's letter begins by describing that arrival and immediate action:
Seeing it is a righteous thing with God to recompense tribulation to them that trouble you; And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels, In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ: Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power;
— 2 Thessalonians 1:6–9
Then the given verse describes those that didn't fight against him:
When he shall come to be glorified in his saints, and to be admired in all them that believe (because our testimony among you was believed) in that day.
— 2 Thessalonians 1:10
A small number (his saints, the elect) had already been called to salvation, and this is the time when they are reborn as immortal spirits (1 Corinthians 15:52), to rule and teach with him during the Millennium that follows.
But there will also be many people that believed (to a certain extent), but who hadn't yet been called by God.
Paul is thanking the Thessalonian church (and all those in the future for which this is a type), for evangelizing, for spreading the word, for creating a large population who have already heard the truth and who will be very receptive to God's word when they are taught it in God's Kingdom.
It is during the Tribulation that many non-believers will suddenly realize, as they see the prophecies happening, that all the nonsense they had been hearing about is actually true. They will marvel at it, and readily accept the truth (thereby making the reeducation of the world that much easier).
Just consider how many people there are today that, though not believing it themselves, are familiar with the concepts of Christianity and the events that are foretold in the Book of Revelation.
Paul is in effect answering this question:
I tell you that He will avenge them speedily. Nevertheless, when the Son of Man comes, will He really find faith on the earth?
— Luke 18:8
The faith of believers doesn't stem on what they have seen, as Hebrews 11:1 reveals;
Now faith is confidence in what we hope for and assurance about what we do not see. (NIV)
Thus on the glorified day of their Lords revelation, believers will marvel to see what unfolds before their eyes. Only those who already had faith ("for our testimony to you was believed"-2 Thess 1:10) will find joy in that moment, echoing Job's declaration in Job 42:5,
My ears had heard of you but now my eyes have seen you. (NIV)
However, for those who did not believe, they will hide in caves and among the rockes of the mountains (Rev 6:15), too afraid to face the unfolding reality.
