1 Thessalonians 3:1 - 3

"So when we could stand it no longer, we thought it best to be left by ourselves in Athens. We sent Timothy, who is our brother and God's fellow worker in spreading the gospel of Christ, to strengthen and encourage you in your faith, so that no one would be unsettled by these trials. You know quite well that we were destined for them." NIV translation

The trials that Paul speaks of are described in Acts 17 where things got so bad in Thessalonica that Paul had to leave the city in the middle of the night. Now, he is saying that they could not stand it any longer but just had to know how the believers there were doing. He sent Timothy to check out the situation in Thessalonica and to bring back a report of the body of believers. Paul also says that the trials that they had experienced there had not taken God by surprise.

1 Thessalonians 3:4

"In fact, when we were with you, we kept telling you that we would be persecuted. And it turned out that way, as you well know." NIV translation

Paul is reminding them that he knew that the persecution was going to happen because that is what happens when you follow Christ. The same is true for us today as Jesus said that we would be persecuted as well.

1 Thessalonians 3:5

"For this reason, when I could stand it no longer, I sent to find out about your faith. I was afraid that in some way the tempter might have tempted you and our efforts might have been useless." NIV translation

Paul knew that persecution was coming and that the new believers in Thessalonica would face trials. He was concerned that the devil would snatch away their seeds of faith as described by Jesus in the Parable of the Sower (Matthew chapter 13).