'Church,' Never mentioned in the OT but 111 times in the NT. Why? And that is giving the OT an approximate 66% handicap.
The original assertion is incorrect.
The word "
church" is mentioned in the OT, it's just not the Greek word "
ecclesia." When the Jews translated their scripture (the Tanakh, or what we call the "Old Testament") into Greek they translated the Hebrew word for the "
assembly" (Heb.:
qahal) using the Greek word, "
ecclesia." The New Testament writers, all of whom were Jews, used that word to describe the assembly of those following Jesus. This is one of the many reasons the early follows of The Way (the way of Christ) were persecuted so violently. The apostles were implicitly asserting it was the followers of Christ that were the
qahal, the true assembly of God, and not the old-line Jews, especially not those following the Sadducees. It was the followers of Christ who were among those who were "
called out," those whose ancestors included Noah, Abraham, Isaac, Jacob, Isaiah, Jeremiah, etc.
Exodus 12:5
You shall keep it until the fourteenth day of the same month, then the whole assembly (qahal)
of the congregation of Israel is to slaughter it at twilight.
"
Qahal," and its conjugated variations occurs about 70 times in the OT (although not all uses are references to God's people). The English word "assembly" occurs 117 times, but not all of them are "
qahal."
This may be part of the reason Gamaliel placated the Jewish leaders, whose power was couched in the Sadducees. Gamaliel, who was of the Pharisaic school within Judaism, believed in a resurrection, and afterlife. While he was not a convert to Christ, he was, apparently, tolerant, if not sympathetic, to the teachings of Christ. The biblical record shows only Pharisees converting to Christ; never Sadducees. I'm speculating, but a Pharisee, Gamaliel, arguing in the assembly

of Council and Senate of the Jewish leadership who all thought of themselves as the
qahal/ecclesia, "
Guys, don't worry. If Jesus is just another false prophet, then those nutcases will drift apart under the weight of their own lie. If Jesus is not a false prophet, then it's best not to get in God's way," makes perfect sense - especially from the perspective of Luke.
This is also why some among us in Christendom, like the Dispensationalists, Christian Zionists, and those prone to Judaizing Christianity get their ecclesiology wrong. Scripture itself overtly asserts a continuity and common identity between the Hebraic assembly and the Christian called out. The assembly is not the only place his is witnessed. The OT also repeatedly asserts characteristics and/or attributes of God's people in the OT that are later applied to the Christian saints.