Paul and his team had no doubt about the authenticity of the church in Thessalonica.  They were brothers and sisters because they had been born again into the family of God through faith in Christ.  They were marked by the Holy Spirit, powerfully transformed to love Jesus and all that is precious to Him (Ephesians 1:13).  That was the result of God’s love for them, and their welcoming the gospel message from Paul’s apostolic team.

God's Word was not presented in a vacuum.  God's servants, Paul, Silas and Timothy (1 Thessalonians 1:1) were living in a way which validated the gospel message which they brought.  The gospel they preached to the Thessalonians was the same message that had transformed their own lifestyle.  And that combination of Word and works, with the Holy Spirit’s power, had become a great model for the new believers.  In their own turn, the Thessalonian church had become a model for other churches to follow (1 Thessalonians 1:7).  This was authentic faith.  So much so, that Paul observed that this new church was living like the believers in Judea who they had never met (1 Thessalonians 2:14), because the same Lord was changing them all to be like Himself (2 Corinthians 3:18).  
 
The apostolic team believed the gospel, lived the gospel and demonstrated the power of God in their lives.  Inspired by the Holy Spirit they presented a compelling joyful package of belief and behaviour which the Thessalonians wanted for themselves.   They did not just copy the way the Apostle lived; they put their trust in the same Jesus and were filled with the same Holy Spirit.  They were not deterred by persecution or threats that worse lay ahead of them (Philippians 1:29). Indeed, they also became full of joy which radiated to the city and beyond (1 Thessalonians 1:7-10).
 
The world does not need more good ideas.  It needs to be convicted of sin by the Holy Spirit and saved through believing the gospel message, trusting in Jesus.  When that message is accompanied by people whose lives have been transformed by the Holy Spirit, others take notice.  Some will welcome the message and submit to Jesus (Titus 2:11-14).  That should shape our understanding of what it means to be followers of Jesus: to understanding the gospel clearly, to repent and receive our Saviour as Lord, to allow His Spirit to fill us with joy and to learn to live His new life without fear.