Paul has not met most of them. Yet there are two things that help us to understand why Paul calls them brothers: a) in the second half of verse 2 he refers to God as “our Father”; b) in verse 7 we learn that these believers came to know Christ from Epaphras who was with Paul and told Paul about them.
By saying “brothers,” Paul acknowledges the bond he has with the Colossians. It is not based on physical things to start with; it has all to do with God being their Father. They all share the same Father and they are all part of God’s family. Truly they are brothers.
2 To the saints and faithful brothers in Christ at Colossae: Grace to you and peace from God our Father.