For not only has the word of the Lord sounded forth from you in Macedonia and Achaia, but your faith in God has gone forth everywhere, so that we need not say anything. (1 Thessalonians 1:8 - ESV)
As mentioned in the previous verse, the members of the Thessalonian church have become examples for this entire area. Their Faith in God is being demonstrated in such a way that Paul doesn't need to say anything to begin the work. I don't believe that Paul doesn't literally need to say anything at all, the gospel still needs to be demonstrated in Word, but their example has started the conversations that need to happen.