Paul’s introductory words in his second letter to the Thessalonians is typical of his letters in that it contains an invocation to God on their behalf, “Grace to you and peace from God our Father and the Lord Jesus Christ.” (1).
There are two things to note in these first two verses. First, the inclusion of Silvanus in the salutation. The name is found only here, in his first letter to the Thessalonians, 2 Corinthians 1:19 and 1 Peter 5:12. However, the man is readily identified by the Greek variation of the name, Silas, one of Paul’s travel companions.
The more interesting phrase is “the church of the Thessalonians in God the Father and the Lord Jesus Christ.” It clearly indicates that such phrases are not proper names, but descriptions of a group. This group lived in Thessalonica, and were “in” God and Jesus. This means they had a relationship with the Father and the Son. This was a congregation of Christians.
This is the biblical use of the term church, and does not in any way coincide with the denominational use of the term in our time.