I assume you are referring to the common definition of the "church" (Gr. ekklesia) as a "called out assembly." The word doesn't really mean that in NT usage, though that is the etymology (word origin). But the meaning of a word is almost never based on its etymology. To make etymology equal the meaning is called the "root fallacy" in linguistics, or sometimes the "etymological fallacy," defined as "The notion that the 'true meaning' of a word is the one to be expected from its etymology" (P. H. Matthews, Oxford Concise Dictionary of Linguistics, p. 128).
The church in the NT has a number of characteristics that no assembly in the OT has: meeting on Sunday, doing evangelism because of the Great Commission, regenerate membership, baptism, the Lord's Supper, local assemblies, etc. The OT saints therefore cannot be part of the church.
However, "I will build" is future active indicative. In other words, the future tense means that when Jesus said that He was not yet building His church.
Sorry, but w Cor 10 teaches nothing about the church. The word is just not there. If we are going to talk about ecclesiology, shouldn't the passage at least have the word "church," or "body of Christ"?
And in 2 Cor. Paul wrote in v. 32, "Give none offence, neither to the Jews, nor to the Gentiles, nor to the church of God." So Paul classified the "church of God" as a separate entity from either the Jews or the Gentiles.