Acts 14:21–22 (NASB95)
21 After they had preached the gospel to that city and had made many disciples, they returned to Lystra and to Iconium and to Antioch, 22 strengthening the souls of the disciples, encouraging them to continue in the faith, and saying, “Through many tribulations we must enter the kingdom of God.”
The background of this Scripture has to do with the persecution of the church. The evangelistic missionary efforts of Paul and Barnabas to the Gentiles was a huge success. But later, when they returned to the region to encourage these new believers to remain faithful, they understood a critical principle about faith; IT WILL BE TESTED.
The testing of faith through tribulations validates its genuineness. We are saved by faith; without it, we cannot please God nor enter His kingdom. So, remember, when persecution, trial, temptation, and tribulation come, it helps us to see the reality of saving faith.