When you get into espousing bad theology, contradictions are commonplace. Many persons will read right by the contradictions, having latched onto the argument you're trying to make, but they're there nonetheless. That's because truth is a powerful thing, and no matter how hard you try, you just can't get around it.
In the issue at hand, one of the major contradictions that arises in the argument presented by the author of this particular book is that the church is entirely a new thing, being an assembly of the people, whereas the Old Testament itself is about a people, not individuals.
Yes, at this point, you should be confused. Let me walk you through how this plays out.
The author claims that it's an entirely new thing that God would focus on the assembly of the people in the New Testament (the most accurate translation of the Greek word we often translate "church"). Prior to this, he says, the emphasis was on the Temple, where sacrifice and ritual were happening and where the presence of God dwelt. It's cataclysmic in terms of what God is doing that God took the emphasis off of the Temple and put it onto the people.
At the same time, he continues to build an argument against even needing the Old Testament at all by declaring that the Old Testament had nothing to do with individual faith, that it was all about the people of God, collectively, as a group. It was about Israel, a whole nation called out to worship Him. So if you're looking for something meaningful in your faith, you won't find it in the Old Testament because all of that was for a group of people, a community.
Whereas, of course, the emphasis in the New Testament is the assembly, the community...a...group...of...people.
Are you catching the difficulty of such an argument? How can it possibly be both?
The inescapable truth is that it can't, and it isn't. God's focus on the assembly in the New Testament is not fundamentally different than His emphasis on community in the Old. It's not something "radically new." In fact, even the assembly of the people as a community was a central feature of the Old Testament.
The people came together often. They came together for festivals and for sacrifice, for remembrances and for atonement. They came together at appointed times and sometimes, just by sheer freewill. In fact, when God built the Tabernacle in the wilderness, He designed it with an outer courtyard for just this purpose - for the people to assemble. When Solomon built the Temple, he built into it a front porch - as any good southerner knows, the place where people assemble. Yes, there were priests and rituals and animals being sacrificed, but there was sacred space for the assembly, too.
And the bulk of Old Testament law, the overwhelming majority of it, has more to do with how to live with one another - as a community - than how to live before God - as a worshiper. The laws set Israel further apart as a group, as a nation, as a people of God.
So there's always been an assembly, and God has always been into it. It's always been the way He wanted to do it. And even the guy that's trying to argue that it's something radically new can't get away from the fact, even in his own arguing, that it's actually beautifully old.