By Mal Couch…The church and Israel did not begin at the same time and are, therefore, not the same entity. The nation of Israel essentially began when God called Abraham and promised to make a great nation from him.The rest of the Old Testament records the growth, development and existence of that nation. The Church did not begin in the Old Testament and, therefore, it and Israel are distinct. The unique character of the Church supports the distinction. The church, unlike Israel, is declared to be a “mystery” (Eph. 3:1-12; Col. 1:26-27). In the New Testament a “mystery” is a truth that was not revealed previously in the Old Testament.
If the Church began at Pentecost, then it did not begin or exist in the Old Testament. It is worth noting that in Matthew 16:18 the Lord Jesus used the future tense: “I will build My Church.” He did not say, “I am building My church” or “I have been building My church.” The church was something still future in Christ’s ministry, which means that it was not in existence during His ministry or in the Old Testament. The apostles would not have understood what He meant by “His church” being built in the future, but the details about the church would be given to them later. In dealing with the matter of the discipline of an individual (18:17), Jesus told them to tell it to the church or assembly. The apostles would have understood that He was speaking of a Jewish assembly. The statement of 18:17 must be understood in the light of the previous statement (16:18) of the future building of “My church.
”Certain things had to be true before the church could come into existence. First, according to the Apostle Paul, the Church is the “body of Christ” (e.g. Col. 1:18, 24 and Eph. 2:16; 3:6; 5:23, 30). It is clear that the church (the body) could not exist and function without its Head, the Lord Jesus. Jesus did not assume that role until after He had shed His blood on the cross, had been resurrected, and then ascended back into heaven. It was at that time, after those events, that the Father “put all things in subjection under His feet, and gave Him and head over all things to the church, which is His body” (1:22).
From:
http://www.whyisrael.org/2010/11/22/the-church-is-not-israel/Defacto