The church in Thessalonica had come to Christ through the gospel message about Jesus, who rescues people from God's wrath (Romans 5:9).  Paul and his team brought the gospel to the city.  As people believed, their transformed lives and joy propelled the news about Jesus throughout the whole region.  It was organic evangelism: good news spreading like wildfire as the seed of truth was being scattered widely by those who had received it.
 
This passage demonstrates the nature of true conversion.  Firstly, it was all based on the apostolic gospel - there is no other true gospel (Galatians 1:6-9).  Paul had been given Christ's direct commission to take the gospel to the Gentiles (Romans 15:15-16).  Their willingness to receive both the message, and its messengers, showed that they were responding to the Word of God (1 Thessalonians 2:13).  Secondly, they made a radical decision to abandon their former religious life and stop worshipping idols.  That choice invited opposition from friends and family, but it also opened the opportunity to explain what they believed about Jesus.
 
Thirdly, they intentionally turned to serve the true God.  Many people claimed to worship a god, but the apostles taught that God is defined as God the Father of our Lord Jesus Christ (2 Corinthians 1:3).  The only true God is intimately related with Jesus His Son.  Fourthly, they had an eternal perspective.  They understood that life on earth is only temporary; it will all come to an end when Jesus returns, and they were willing to serve Him faithfully until that day.  Fifthly, they understood that their sins could only be washed away through the sacrifice of Jesus, so that they would be blameless when Jesus comes as Judge (1 Peter 4:5).
 
In a world where people believe many ideas, it is important to understand that God the Father is best known and understood by His relationship to Jesus Christ (Hebrews 1:1-3).  His death is the only atoning sacrifice for sins and the only protection against the Father's wrath about our rebellion (1 John 4:10).  Believing this apostolic gospel brings about true conversion: a change on the inside which expresses itself in turning from idols to serve the true God, who we cannot see, but for whom we wait.  May the Lord enable us to be that kind of witness of the grace of Christ.