Thank God for the believers

8 First of all I thank my God through Jesus Christ for all of you, that your faith is spread throughout the whole world.
9 For God is my witness, whom I serve in my spirit in the gospel of his Son, who without ceasing I make mention of you always in my prayers,
10 praying that somehow you have at last, by the will of God, a prosperous journey to go to you.

Paul lived at a time when the phrase, "all roads lead to Rome," was a real fact rather than a simple expression. Rome was the capital of the world. In this turbulent and decadent capital there was a flourishing church of Christians, probably founded by people who were in Jerusalem on the day of Pentecost where "visitors from Rome witnessed the manifestation of the Holy Spirit, Paul had never been to Rome, but the book to the Romans in the new testament in his letter to that church.
Paul thanked God for these people.

Source
The apostle Paul gave thanks to God for the believers, because their faith was a topic of conversation literally in the whole world. Travelers passing through Rome noticed in Christians a new form of faith, a new way of living, and a new way of approaching God personally.

This captivated people's attention and made them listen and this is what the Holy Spirit can do in us. His presence in our lives can make people want to know what makes us more loving, more hopeful, more peaceful, more fruitful. We need to ask God daily for a fresh flow of his holy spirit in our lives, which serves as a refreshment for those around us, to the point that makes them want to know Him!