The Holman Bible Dictionary, in its article "Church," explains the background of the word church (emphasis added throughout):
"Church is the English translation of the Greek word ekklesia. The use of the Greek term prior to the emergence of the Christian church is important as two streams of meaning flow from the history of its usage into the New Testament understanding of church.
"First, the Greek term which basically means 'called out' was commonly used to indicate an assembly of citizens of a Greek city and is so used in Acts 19:32-39. The citizens who were quite conscious of their privileged status over against slaves and noncitizens were called to the assembly by a herald and dealt . . . with matters of common concern. When the early Christians understood themselves as constituting a church, no doubt exists that they perceived themselves as called out by God in Jesus Christ for a special purpose and that their status was a privileged one in Jesus Christ (Ephesians 2:19).
"Second, the Greek term was used more than one hundred times in the Greek translation of the Old Testament in common use in the time of Jesus. The Hebrew term (qahal) meant simply 'assembly' and could be used in a variety of ways, referring for example to an assembling of prophets (1 Samuel 19:20), soldiers (Numbers 22:4), or the people of God (Deuteronomy 9:10). The use of the term in the Old Testament in referring to the people of God is important for understanding the term 'church' in the New Testament.
"The first Christians were [mostly] Jews who used the Greek translation of the Old Testament. For them to use a self-designation that was common in the Old Testament for the people of God reveals their understanding of the continuity that links the Old and New Testaments. The early Christians understood themselves as the people of the God who had revealed Himself in the Old Testament (Hebrews 1:1-2), as the true children of Israel (Romans 2:28-29) with Abraham as their father (Romans 4:1-25), and as the people of the New Covenant prophesied in the Old Testament (Hebrews 8:1-13).
"As a consequence of this broad background of meaning in the Greek and Old Testament worlds, the term 'church' is used in the New Testament of a local congregation of called-out Christians, such as the 'church of God which is at Corinth' (1 Corinthians 1:2), and also of the entire people of God, such as in the affirmation that Christ is 'the head over all things to the church, which is his body' (Ephesians 1:22-23)."