Were all the Old Testament Israelites true believers?
No. Most Old Testament Israelites were not true believers; they were renegades in unbelief. They had the opportunity, the Word of God, and everything going for them, but most still did not believe. They remained on the root of Abraham even while in unbelief.
Les explains that Israel was in the promises given to Abraham, but God had to set Israel aside, breaking them off from the fatness of the root of Abraham, so the Gentile system could be brought in. Yet this did not mean all those Israelites were believers. They had the opportunity, the Word of God, and everything going for them, but most were not true believers; most were renegades in unbelief, though still on the root of Abraham. Likewise, the Gentile system today rests on the root of Abraham, not becoming Jews but feasting on what God promised Abraham. Gentiles have just as much opportunity at salvation and a relationship with God as Israel did.