"For this cause also thank we God without ceasing, because, when ye received the Word of God which ye heard of us, ye received it not as the word of men, but as it is in truth, the Word of God, which effectually worketh also in you that believe. For ye, brethren, became followers of the churches of God which in Judaea are in Christ Jesus: for ye also have suffered like things of your own countrymen, even as they have of the Jews: who both killed the Lord Jesus, and their own prophets, and have persecuted us; and they please not God, and are contrary to all men: forbidding us to speak to the Gentiles that they might be saved, to fill up their sins alway: for the wrath is come upon them to the uttermost."
In this passage of Scripture, notice that there is a natural progression that follows one's faith in the Gospel, which is the power of God unto Salvation (Rom. 1:16; 1 Cor. 15:1-4). The Apostle Paul had proclaimed Jesus to those living in the city of Thessalonica, while there on his second missionary journey (Acts 17:4). Many had recognized his message as being the Word of God, and not just Paul's opinion; as a result, they believed and were saved (v. 13)!
How did Paul know that they had really been born again? Because, they immediately began sharing the Gospel with others, and as a result, they experienced persecution from those who had also rejected Paul's preaching (v. 14). Paul encouraged them by telling them that they were experiencing the same reaction as did the Apostles in Jerusalem; the Jews tried to silence the preaching to the Gentiles there as well (v. 15-16).
Paul was reinforcing what he had written in the first chapter of his epistle; Paul had stated that when he had come to them, his preaching had been empowered by the Holy Spirit, and that his life was consistent with the message He preached (1 Thes. 1:5). As a result, they had become followers of Christ, with Paul as their role model (v. 6). And, just as the unbelievers in their city had persecuted Paul, they too were experiencing rejection. But, because they exhibited the Spiritual fruit of joy in the face of opposition, they became role models for others in Macedonia and Achaia (v. 6-7)!
Yes, there is great joy in sharing one's testimony with others! But you know what gave the Apostle Paul even greater joy? Receiving word that those whom he had led to faith in Christ, were sharing the Gospel with others (2:19-20)! Those who led me to Christ are filled with joy when they hear I have been used of the Lord to win others!
It is like a grandpa rejoicing over seeing his son lead his grandson to Christ!