was there a "church" in the old testament?

Dispensational and New Covenant theology say NO. Covenant theology says YES.

Herman Ridderbos speaking on Matthew 16:13-20: "We should note that this is the first and, besides 18:17, the only time that Jesus spoke of His church in the Gospels. He apparently was alluding to the time when the disciples would carry on His work in His name. He referred to the future fellowship of believers with a word that corresponds to the name that had been given to ancient Israel as a religious community (Hebrew: qahal). Commentators have argued for a variety of reasons that Jesus could not have spoken of such a future organization at this stage of His life. Thus they think that the statement was not Jesus’ own but was put back into His mouth later by the church. Such a glance into the future is not at all surprising at this point, however, since the gospel here is taking a distinct turn toward the end of Jesus’ life. What is more, Jesus did not speak of an organized church. He was merely alluding to the future community of those who would believe in Him. In view of what He said elsewhere about "fishers of men" (4:19), about His "sheep" (John 10:15; 21:15) and his "flock" (John 10:16), and in view of the close circle of disciples that He had gathered around Himself, His notion of such a community cannot be considered anachronistic.

By calling this community His church, He designates it as the people of the Messiah, the community that would replace Israel as the people of God. Jesus had more to say about this future church: `the gates of Hades will not overcome it’ . . . . The church of Christ . . . will not be overcome by this power of death. Jesus spoke here as one who was stronger than death and who would cause His church to share in His victory over it." [See Herman Ridderbos, Matthew: Bible Student’s Commentary, trans Ray Togtman (Grand Rapids, Zondervan, 1987), 304].

While the church is clearly a new covenant institution [Cf. Meredith Kline, The Structure of Biblical Authority (Grand Rapids: Eerdmans, 1981), 193], Jesus’ use of the word qahal clearly indicates a continuity with the Old Testament people of God. The connection then is eschatological and covenantal [see Donald A. Hagner, Matthew 14-28: Word Biblical Commentary, Vol. 33b (Dallas, Word Books, 1995), 471].

Now, would I be emphatic about this point? Not too much, but it is important to realize - whichever side of the fence you're on - that God's elect throughout history have always been save by grace, through faith in the historical Messiah.

See a comparison of these 3 systems: Disp, NCT and CT