The word Church (ekklesia) is only used in the New Testament. In the Old Testament, the word congregation (adar) is used. If the same thing is being referred to in the Old as well as the New, then why only use Church in the New and congregation in the Old when translating to English?
One possible answer: The English translators (under the influence of gentile ‘church’ fathers) deliberately used the word in the New but used a different word in the Old to make a separation between Israel and the new covenant brethren that was never there.
There’s no question that there was an effort to distance the Church from Israel in the 3rd and 4th Centuries. But I think in this case it’s a little simpler than that. In what’s most likely the earliest written communication to believers. Paul, a Jew, wrote to the Church (ekklesia) of the Thessalonians rather than the congregation (Hebrew: sunagogue) of the Thessalonians.
In fact, the only appearance of the word congregation in the New Testament is in Acts 13:43, where the Greek word for congregation is the same as the one for synagogue in verse 42 (sunagogue). Since most of the places Paul went to start a church already had a synagogue, he may have used the word ekklesia in addressing the Christians simply to distinguish between them and the Jews, even though there’s really only one church, made up of numerous congregations.