The relationship of the New Testament Church (Greek ekklesia) to the congregation of Israel in the Old Testament can be better understood when we learn the different interpretations placed on the two Hebrew words for "congregation": 'edah and qahal.
The Holman Bible Dictionary, in its article "Congregation," explains that these Hebrew words were used with a significantly different meaning in the days of Christ and the apostles: "In the Greek Old Testament [the Septuagint] 'edah was usually translated [into Greek as] sunagoge, [and] qahal [as] ekklesia. In late Judaism sunagoge [from which derives the word synagogue] depicted the actual Israelite people and [the word] ekklesia [depicted] the ideal elect of God called to salvation. Hence [the Greek word] ekklesia became the term for the Christian congregation, the church...There is a direct spiritual continuity between the congregation of the Old Testament and the New Testament Church. Significantly the Christian community chose the Old Testament term for the ideal people of God called to salvation(ekklesia), rather than the term which described all Israelites collectively (sunagoge)."
This explains why the New Testament word for the Church, ekklesia, refers only to those people, Jews and gentiles, who are called by God to receive salvation through Jesus Christ. Therefore, the Church of God, the term most generally applied to God's people in English translations of the New Testament, is the body of people who are special to God because they obey His Word and accept His Son, Jesus Christ, as the Messiah.